Except unions aren't for-profit, so it's unclear where those extra money is going to. Senior leadership perhaps? But unions beholden to members, which means there's no "fixed percentage", because that's decided by the members themselves. We can see a parallel in public sector compensation: democratically controlled organizations are very reluctant to spend extra money on compensating leadership (or any other high paying staff, for that matter).
The extra money goes to more union staff and union facilities; lobbying; and grants - overwhelming the former. Also, it's not like non-profits have to balance their budgets every year; the UAW added $60M to its net assets in 2021.
