IME, there is usually "just enough" room for everyone's carry-ons in FC on a 757. If boarding through 1L, all it takes is a couple Y passengers to think they can put their bags in the first available bin space they see (and among the 150 or so Y passengers a 757 can hold, there's probably going to be at least one or two who think that way) to cause the FC cabin to run out of bin space.