1995 results are clearly wrong and inversed: it was Chirac who won 60.9% in the second round, not Jospin. A fair number of 1995 second round results at the local level were erroneously inversed in this way. Therefore, the rather sharp shift from right to left in presidential voting happened between 1995 and 2007 (when Ségolène Royal won with 60.9%), and has only gotten more marked since.
Saint-Pierre-et-Miquelon is very much
sui generis in its politics like most other overseas collectivities, and national politics - especially in the past - may have been far less relevant to them.
Still to this day, parochialism - relative dislike of outsiders, longstanding resentment of metropolitan technocrats, strong local culture - seems to be the dominant tone of archipelago's politics. It seems as if the national political parties were not locally organized into federations until relatively recently, and local politics remain dominated by local parties only loosely tied to national parties -
Archipel demain, which controls the territorial council and the Senate seat, is affiliated to the centre/right;
Cap sur l'avenir is led by Annick Girardin, the current minister of the sea and ex-PRG/LREM deputy since 2007.
Local politics have also long tended to be dominated by a small number of political figures, often with distant family ties among themselves (their surnames also reveal the Breton, Norman or Basque origins of the majority of the insular population) - for example, Albert Pen (senator 1968-1981, deputy 1981-1986, senator 1986-1995), Marc Plantegenest (deputy 1978-1981, senator 1981-1986), Gérard Grignon (deputy 1986-2007). All three had fluctuating metropolitan political affiliations: Pen was initially a Gaullist, but became a Socialist in 1974, and remained affiliated with the Socialist group although he quit the PS as early as 1977; Plantegenest, at first a protégé of Pen, sat as a non-inscrit in the National Assembly and gave his signature to Giscard in 1981, but then sat as a Socialist in the Senate, which reveals a certain tendency (in the past, at least) to side with whoever was in power centrally (a trait shared with other overseas places, hi Wallis-et-Futuna!). Grignon was among the founders of
Archipel demain, the centre-right local party, and always sat with the UDF or UMP groups, although in his unsuccessful 1981 campaign he said that he would sit with the Socialist group if elected and support Mitterrand.
There's also tended to be disconnect between voting patterns for different levels: as a recent example,
Archipel demain has won the last three territorial elections (2006, 2012, 2017) - with no less than 70% in 2017 - while
Cap sur l'avenir (Girardin) won the last four legislative elections (2007, 2012, 2014 by-election, 2017) - although her 2017 reelection was very close (surprisingly close, at least seen from Paris, given that she was an incumbent cabinet minister).
As an older example, while Giscard won the archipelago by huge margins in 1974 and 1981, left-leaning candidates - Plantegenest and then Pen - won the 1978, 1981 and 1986 legislative elections. Both in their times campaigned as the 'local' candidates, defending territorial interests (at the time in opposition to the departmentalization of the island in 1975, which was locally controversial, in part because of the threat of a huge influx of metropolitan bureaucrats), against 'carpetbagger' (
parachutés in French parlance) candidate from metropolitan France. While Pen didn't hide his sympathies for the left/PS, he largely campaigned as the 'local' candidate; in both 1978 and 1981, Plantegenest and Pen's opponent in the runoff was a UDF candidate from metropolitan France, who tried to campaign with a then-typical 'socialo-communist danger' red scare campaign. As a fun anecdote, the RPR's candidate in 1981 was Julien Lepers - later the host of
Questions pour un champion on France3 (1988-2016), who got trounced, in part because he was not a local. When Grignon won the archipelago's seat in the National Assembly in the 1986 by-election (against Plantegenest I believe), it was in good part because he campaigned against the monopolization of all local powers by Pen's political group.
The trend in presidential voting can, I think, as Estrella mentioned, be explained by the usual patterns of secularization/decline of religious practice/weakening of the religious cleavage seen elsewhere in France. Saint-Pierre-et-Miquelon was a very religious place - I think, as late as the 1980s, a majority of students attended
écoles libres (private, usually Catholic, schools), although don't quote me on that. From its wartime history, Saint-Pierre-et-Miquelon was also a Gaullist stronghold - Jacques-Philippe Vendroux, the nephew of de Gaulle's wife, was the deputy (UDR) between 1967 and 1973 (despite not being an islander and very much a
parachuté).
Historically the archipelago's economy was dominated by the cod fisheries, like for Newfoundland, but the cod fisheries collapsed following the delimitation of France's EEZ in 1992 and the Canadian cod moratorium in the 1990s. Since then, Saint-Pierre-et-Miquelon has been heavily dependent on public sector employment and funding and subsidies from metropolitan France (which have been very generous: the economic situation, post-cod fisheries collapse, on Saint-Pierre-et-Miquelon has been better than neighbouring fishing communities in Newfoundland...). Your guess is as good as mine, but perhaps the dependence on public employment and funding may explain (along with secularization) the bias towards the left in recent presidential elections. In 2017, Mélenchon (35.5%) and Panzergirl (18.2%) placed first and second, and Panzergirl overperformed her national result in the runoff (36.7%, although less than half of voters cast valid votes) - Panzergirl had made a point of visiting Saint-Pierre-et-Miquelon during the campaign (something which I think is rare for any presidential candidate) to sell the image that she wouldn't forget about them. Perhaps the traditional isolation from/resentment against metro France helped Mélenchon and Panzergirl, and hurt someone like Macron.
Edit: for those in Canada, here's a really good documentary about the archipelago:
https://www.knowledge.ca/program/island-diaries/s4/e8/st-pierre-and-miquelon
