I noticed something similar recently, not from a BA outstation but as a result of having my boarding passes issued by AA on SFO-ORD-LHR-EDI, the first segment only being on AA metal. I was dubious about the gate close time shown for the ORD flight, but I don't transfer there often enough to be sure. But I do LHR-EDI often enough to be certain that the time shown for that segment was definitely wrong. In this case, every segment had the same interval shown between gate close (or was it boarding? - can't recall) and scheduled departure. This suggests AA's systems apply a blanket rule with no actual knowledge of actual practice at each airport.

It was also noticeable that the AA BPs just had less information on them than the BA BPs I am used to.