Three years late but it was just pointed out to me that the answer becomes obvious when looking at the vote totals.
1976: 217 R 131 D
1980: 290 R 109 D
1984: 296 R 227 D
1988: 272 R 132 D
Mondale didn't have this stunning improvement over Carter, someone accidentally wrote down 227 instead of 127 and it never got caught.
Honestly, this seems much more likely than such an odd voter increase that was one-sided and only lasted for one election, especially since the increase was one-sided against the candidate that won the state and country in a historical landslide. And if the Mondale vote was actually 127, it would put the two-party vote at about 70 R - 30 D, which is exactly in line with how the county voted in other elections during this time period. Unless there is historical evidence for why this county gained almost entirely Democratic new voters during a Reagan landslide and lost them the following election, I think this was purely a miscount.
