GAs are given a number based on some algorithm/study conducted by DL that determines how many bags can fit overhead bins, they are not usually in contact with the FAs. If the system says 20 bags can fit, they will only let 20 bags on. Some will guesstimate, others will count every large rollerboard that boards.

There is an incentive to get the flight out on time, and it's usually safer for the GAs to be conservative on OHB bags instead of making 20 people deboard and have their items tagged last minute.