Why was the local sheriff so easily fooled?

First fobbed off to by Tom simply telling him that he's not in a witness protection programme, and then by his wife telling him he has too much time on his hands or something along those lines, when the sheriff that time had more grounds to be suspicious.

I mean for starters Tom would have moved to that area fairly late on. Secondly, wouldn't a simple background check have cleared things up? Maybe being in a WPP would hide information to the local authorities, I'm not sure how it works. But the third and most obvious reason for doubting who Tom is who he says he is, is that he took out 5 armed killers in separate incidents, when he's supposed to be this typical everyday guy.