Except, a gallon is a gallon no matter which gas station I'm at. Also I know my car's gas mileage, and it doesn't change when I visit a Shell station instead of a Chevron. The composition of the gas is regulated, as are the pumps that dispense it. There are inspectors from the state whose job it is to ensure that when I buy a gallon, I really get a gallon.

Tokenizers aren't standardized to anywhere near that level. A "token" from one isn't the same as a token from another.