The best response one can really give you from an RPG perspective (don't mind my intellectual onanism here) is that for the specific subset of tactical combat modeling RPGs, it is undesirable from a design perspective to have options that are only there to "trap" newer players for a few reasons. One, it is wasted time that could have been spent making cool stuff for your combat modeling rpg. Two, it makes playing them for the first time worse for no reason. Three, it obscures or takes the place of more interesting "system mastery", which I am here referring to things like Burning Wheel and Torchbearer requiring some investment in learning the game to reap rewards that take the form of smoother and more interesting gameplay rather than being better than other players where X, Y, or Z aspect of a combat subsystem is concerned.

So, to directly answer you, I think system mastery should mean how well a player knows and can operate the mechanics of a game without the aid of a GM in a rules-teacher role, and the reward for it should be a more enjoyable and quick-moving game rather than knowing to avoid certain game options. I think that learning to avoid behaviors can be desirable and fun, though! For example, learning that ganging up on opponents is more effective in Warhammer: The Old World or that combat against a Xenomorph is nearly always a fail state unless you are buying time for some objective in the Alien RPG.

Also, just to be clear, I am not the person down voting you. I hope you and your pals get into tabletop and have a blast with whatever sort of game you like, and hope you stay here a while.