This is quite common practice, called "codesharing". I'm sure there is some terribly clever marketing/ business reason behind it all (the marketing carrier, in this case, AY, gets a slice of the cake -- they haven't sold all their allocation yet, so may have cheaper fare buckets to sell than BA).

In terms of flights, of course, there is no issue, but in terms of frequent flyer earnings, there can be a difference for accrual etc.

tb

PS some clever bod will probably come and explain it with far more technical jargon (and possibly clarity) in a minute.