Normally BA redemption are cheaper than AA redemption within BAEC. However, when flying on "Peak" days, AA and BA are the same price. Hence, if you are flying on a peak day, you can choose any Oneworld airline for the same price. During off peak redemptions, BA reduce the Avios cost for their own flights. So on LHR-JFK return, BA will charge 24,000 Avios on off-peak days. Meanwhile on peak days BA charges 40,000 Avios (as is the example in your requested flights. Oneworld partners are always charged at the peak rate.

I'll also point out to you, because it looks to be potentially relevent, that domestic add-ons to longhaul journeys are included in the price (avios wise). I.e. You could flying INV-LHR-JFK return, for the same Avios cost as LHR-JFK. Incidentally, if you begin in INV you save the Air Passenger Duty charge, which can be significant (especially if not flying in Y).