> [...] 90 percent of overtime is voluntary

It's likely "voluntary" in the same sense that arbitration clauses are "voluntary" in many contracts. That is, you voluntarily agree to arbitration clauses in a contract because those terms are not mandated by statute to be in said contract. They are not voluntary is the sense that they're optional! You absolutely have to agree to those terms as part of the contract.

Similarly, voluntary overtime likely just means the overtime itself was not initially stipulated hence by definition it's voluntary, rather than referring to whether agreeing to do overtime is voluntary.

Aren't Equivocation Fallacy fun!