When flying WT+ from LHR, I've heard a number of reasons why I can't use the FBD a minute earlier than T-3h before scheduled flight departure. 'The tag won't read properly; the bags will go in the wrong bin; the baggage handlers need to clear earlier flights; your bags may not make it on the flight with you' (this last a not so veiled threat) - I've heard these and more.

When flying in J, checking in at T-4.5h was 'not a problem, here are your passport and boarding pass.' It's just not been worth my time to question the discrepancy - I say thanks, then go to the lounge for a glass of forgetfulness.