Some airlines will just let you get by with 3 pieces even though it clearly states that you are only allowed 2 pieces on your ticket. Oneworld partners do not have the extra baggage allowances that each particular FFP allows you in their own programme. As mentioned above for QF. The same goes for CX, Gold and Diamonds (Sapphire and Emeralds) are allowed one extra piece on CX transpacific flights only.

AA's Exec Plat does not even have this benefit, nor does BA I think. So if you have been able to get away with it so far, count yourself lucky. But you might be charged should an airline be a stickler and abide by the rules.