I'd be astonished if that's how it actually happened Libertine, as it doesn't make sense. If it were the case then technically you're not competing against the candidates from the other schools on the final selection day. And by this reasoning if the standard was a lot lower in one group than in another, BA would be turning away superior candidates in favour of inferior ones purely on the basis of splitting the allocation between the FTOs. Not gonna happen...
But I agree with you; it's best to choose a school based on other reasons rather than the slight chance of it increasing the probability of you being selected. Because ultimately you'll be up against the best from all schools at the final day.