Steve doesn't know what Tony means by letting him have something. As far as he knows, he's just doing things he wants to do, if that benefits Tony, all the better.

And if he means letting Tony have the credit for just being a friend to him - well, maybe Steve's gotten the impression from the other one that being friends with Steve is more difficult than it appears on first glance.

"Can't expect me to understand newfangled institutions like math," he responds instead.