That's not always true. During the Revis days, sometimes he'd play against the #1 guy straight up, sometimes he'd cover #2. A lot of it was based on match-up.
He was on AJ Green straight-up a lot during the Bengals game, but then he wasn't on Calvin Johnson against Detroit (Browner with McCourty combo called upon there).
And last year, Butler was by far our best cover corner, and played against Antonio Brown a lot in the Pittsburgh game. But he didn't play against Julio in the Super Bowl, again, because of match-up.
During all those games, there's obviously mixes in terms of coverage and you almost never see one corner follow one receiver the entire time, although we have seen that at times with Gilmore this season. And maybe Gilmore will be matched-up exclusively with Brown, it's possible. But I'm just saying it's not as simple as assuming your best cover guy goes on their #1 target every single time.