PFF's coverage stats on a FS can be very misleading. I don't know that I would trust them. I do tend to look at the numbers of the CBs and of the SS against the TE because those are single cover draws but the FS doesn't usually draw a guy in single coverage much, his job is to cover over top and help, and PFF never reflects those instances.
Check the coverage stats by position for all safeties that played 50% of snaps or more. Of the 14 guys that had 40+ passes thrown against their coverage, only 4 were free safeties.
It's just a trick in the way that PFF accumulates their stats. What they're accumulating for a free safety is only a small percentage of what a free safety should do in coverage, and doesn't reflect very well his value in coverage.