The reason the mayor said "an even number or zero" is because if he said "an even number", it's really obvious that many people with zero ending license plates would show up both days claiming zero was neither even nor odd, thinking they had found a loophole and/or that zero plate holders were lucky and special.

Explicitly including zero in the rule is not an assertion zero is not even, it is an attempt to head off the confusion, controversy, lawsuits, fights, and wasted time needed to later modify the rule when the inevitable controversy arises.