Earlier quoted context omitted.
Larger unions sometimes hire staff to represent their interests who are do not work in the profession represented: negotiators, lawyers, etc. It sets up a bit of a principle/agent problem, but it also makes a lot of sense: a professional lawyer may be much better at negotiating on your behalf than the people from your profession who win the union elections.
At least in theory unions have the correct incentive structure, as they get a fixed percentage of their member's incomes, so to get more money from their members, they need to negotiate better salaries for them.
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.