I'll try to provide some insight on this, even though it was not a NFL level event by any means.
When I was coaching as the D coordinator, the best player on D was my MLB. Undersized but aggressive as all get out, nonstop motor, always found a way to anticipate were the ball was and get there ahead of time.
So we have a game at the end of the season that we have to win to make the playoffs. The MLB came down with the flu. He suited up but there was no way I was going to start him. He had no stamina and wasn't 100%, so that put him at risk IMO. He was nauseous and a bit shaky, but we did suit him up. He was on the sidelines the whole game, did not play a down. We were up by 4, opponents have the ball on their last possession, and started driving. They finally get to our 30 yd line with about 2 minutes left and they have solved our D.
The MLB was driving me nuts wanting to play. He hadn't barfed all game, was active and encouraging the whole game on the sideline - mostly in my ear wanting to play. So given the limited time frame and that he was apparently healthy enough to take a few snaps, in he went. Sure enough, two plays later he sniffed out a screen, stripped the ball and recovered the fumble. Game over.
True story - local paper had a huge write up over the whole thing. There was no way I was planning on playing him and wouldn't have played him a down under any other circumstances.
That's why you see guys active and sometimes they don't play much or at all. They are borderline healthy enough, but coaches don't want to risk playing them under anything other than very limited and special circumstances.
Long post, I know. And I know it's not about the NFL specifically, but maybe it will give you a little insight as to why things like this can happen.