It's called yield management and overselling. It looks like BahamasAir is paying more to AA for the RDU-MIA sector than AA/Eagle is, so AA is willing to take a greater risk of overselling. Another way to look at it is that BahamasAir has a lower fare for its sector than AA/Eagle. Or their contract with AA has some other compensation clauses that cause AA to be willing to oversell for BahamasAir when they won't oversell for AA.