Hidden (?) units.

[Schoenwulf]

So Webster says that a game is " a physical or mental competition conducted according to rules with the participants in direct opposition to each other". One of the operative terms there is "rules", which should the same for both players. One sets their strategy in this case  on the this assumption. So when one is in a game and their opponent doesn't follow the rules, it's no longer a game, but a contest of which player can get away with the most egregious violations without getting called on it, sort of the game within the game. The term "rules lawyer" is a rather silly oxymoron, since any disagreement on the rules can be quickly settled without the need for litigation simply by a die roll.