Yeah, but when I said "smallish" I was thinking more on the order of 1%. I would consider 10% actual gains to be quite large given the craziness of what Hyperthreading tries to accomplish.
It may also be a matter of more fully utilizing multiple integer/floating-point units. Say, if the CPU has two integer units but the current code is only using up one of them, then it could run the second hyperthread on the other. I really don't know the details though.