Its pretty much this. Its not a surprise that most of the undecided people at DO schools go into PC. I mean if all your mentors are PC physicians, you're required to do x number of additional cores in FM, primary care, ambulatory, and most of your affiliated residencies are FM and IM, is it really a surprise at all if you end up thinking its something you're interested in?
My school has FM and Geriatrics rotations required during 3rd year, and has 2 ambulatory rotations, a primary care selective and a rural/underserved rotation done with an FM doc during 4th year. Its fair to say that we get a better picture of and more exposure to FM and PC in general than some of the MD schools around here that don't even have an FM program in house, where some do the rotation just with general IM docs, and some that don't even have a required FM rotation in 3rd year. The same can be said about the MD school in the state that focuses on PC and has many PC/FM oriented rotations. Its not a surprise that on average they send more of their grads to FM and PC than other MD schools.