Joke poll.
Carson is not leading in any state at this point, other than maybe in MD.
Is it really that hard to believe though? Considering that basically nobody cracks 20% outside of their home state, it's not too much of a stretch to think he could lead in a few random states despite having support in the teens.
Except for Romney. He's at ~20% or more in the national polls, and we've had polls in the last six months by other pollsters that showed Romney leading in CO, IA, KS, MA, MI, NH, NC, and even AR. The only place where he's trailing is in NJ, where Christie is of course leading. This is the first poll outside NJ that includes Romney in which he's trailing, and it seems rather unlikely that he'd put up such a poor showing in PA of all places.
Sometimes I think Pennsylvania's status as a Northeastern state is really overstated for Republicans. There is little reason to expect Romney or Christie to do well when the entire eastern part of the state is full of heavily Democratic voters. There are some Republicans in the western Philly suburbs, but the Democratic strength extends all the way up to Scranton pretty convincingly. Republicans are much more Midwestern types either in both the central and western parts of the state.
The one thing that sort of contradicts that would be the stereotype of pro-choice Republicans, but my hypothesis would be undue influence from the eastern Republicans for in-state candidate recruiting. These candidates are all outsiders.
Of course, if Romney is at 22% nationally, you'd probably expect 17% or so here, but that's not extremely off from what he received. It's so late in the primary season, PA won't matter unless the field is as splintered as I am hoping for.