Yes.

The underboss (and probably the caporegimes) have to spend a lot of time around the Don, kissing his ass and waiting on him hand and foot. Sure, they get a cut from everyone under them, but the Don takes a big cut of that from them--they're right in front of the Don. And, there's really no way an underboss or a caporegime can keep accurate tabs on just how much everyone under him is earning.

A crew chief, or even a soldier, who spends 100% of his time hustling, theoretically can make more. Sure, he has to kick part of it upstairs, but if he's clever (and ballsy), he can hide most of his earnings from those above.