In a GSP, I bid $1. I'll pay the second price (not really but for purposes of clarity assume it) which was $0.90.

In this case, I always (almost) pay less than my maximum bid, which allows for margin given to me (assuming true value and a bunch of other unrealistic things) rather than in the FPA, where I pay $1, assuming my bid is highest.

I'm not talking in incentive-compatibility terms, rather in terms of where the "margin" goes.