I wrote this as a comment on the post about the signing, but wanted to make sure everyone could see it:
"So I was pouring over the numbers trying to figure out what the bills saw in Williams compared to Legursky. My first instinct was he is 5" taller, so maybe the Bills want to get bigger. But I was digging into the PFF numbers and realized that Williams played against the number 1, 3 and 5 defenses 2 times each. Legursky only had to play against one top 5 team (Kansas city). So that has an affect on the overall PFF ratings for the two if you remove those games:
Legurksy: -12.9 in 698 snaps
Williams: -10.2 in 530 snaps
Now the players look pretty similar. Finally I looked at games against common opponents and this is what I saw:
That means against common opponents Legursky scored a total of -4.2 and Williams a total of +0.6. And that folks might be the silver lining to this signing."