I don't know for sure where all the sites get their information from, but I think that ExpertFlyer's is derived from GDS information, which in turn is AIUI fed by automatic reporting by the aircraft via ACARS or something like that when the parking brake is applied, which is the same source that feeds ba.com's flight information page. It would not surprise me if the other websites that have such information (eg flightstats) get their information from the same source.

So if I were to guess, I'd say that 2229 is likely to have been the official arrival time, ie parking brake application, with doors opening after that time.