It's slightly more complicated in practice. The US has this weird system where you can opt out of full union membership and become what's called an "objector". You opt out of union membership but still pay a portion of dues (those directly around contract negotiation and the like), and you're still covered by the collective bargaining agreement. The downside being that, as you're not a union member, you generally don't get voting rights.

To quote the NLRB's website directly:

> Federal law allows unions and employers to enter into "union-security" agreements which require all employees in a bargaining unit to become union members and begin paying union dues and fees within 30 days of being hired. Employees may choose not to become union members and pay dues, or opt to pay only that share of dues used directly for representation, such as collective bargaining and contract administration. Known as objectors, they are no longer union members, but are still protected by the contract. Unions are obligated to tell all covered employees about this option, which was created by a Supreme Court ruling and is known as the Beck right.

This usually works well enough, but in certain circumstances, you can have a small minority control the entire operation and the vast majority are "represented" by an institution they have no voice in. This seems to be the case in lower paid unions (e.g. grocery baggers) because the majority can't afford the extra dues.

Unions are compensated proportional to N*S where N is number of workers and S is average salary. Workers care about increasing S, but unions sometimes focus on increasing N because it is easier. But then the individual worker doesn't see their life improve.

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).