Possibility 1: Exchange rate differences/roundings, although the likelihood of these amounting to GBP 10 depends on the fares you were booking.

Possibility 2: aa.com had access to a seat in a lower booking class than ba.com, which is sometimes possible as ba.com set to UK residency looks at what seats are available to the UK market, whereas aa.com always looks at the US market which can irritatingly and unfairly have different availability levels.