I amended my post as AA actually does use dual-inventory fares on PE fares to EU. However, another possibility is that it contructed some kind of broken fare option with a connection on the return flights for PE which is not allowed on BE/Main fares. The problem with roundtrip fare quotes is that you don't know if the routing is the same on both the oubound and the return flights for the different fares until you have selected the outbound flights and can see the return options. Some airline sites only quote the pricing on the outbound fares in the initial listing when searching for roundtrip fares which prevents such odd fare combinations from showing.