The Ps4 exlusives we've seen so far weren't that great, hence they received lower scores. Plus, during the Ps3 days Sony exlcusives got inflated scores from fanboy reviewers (in fact, a common factor to all of those games was very high quality graphics and great cinematics that proved Ps3 was superioir machine compared to the 360) .
Now that Ps4 is fimrly in the 1st place, and no one has to prove its superiority over the Xbox One, reviewrs have become slightly more objective agian.