I suspect there's a difference, but I can't prove it.
In 1998, I tried to book a pair of biz class seats on AA/BA using AA miles. 10 months in advance, the trans-Atlantic seats on AA were not a problem for any of several dates I was considering. But the BA Heathrow-Istanbul flights were a problem for almost every conceivable date.
AA reps explained that Istanbul (and Athens) are the hardest BA destinations to secure seats for during the summer. But 10 months in advance?