I know this is actually an old problem but the fact that it still rears its head is annoying. RPGs have long been plagued with the disconnect between story/narrative and game mechanics. The old example of this is a character jumping off a 100' high cliff because the player did the calculations and determined the character could survive the fall. Narratively a person can not survive such a fall, nor would any sane person make such a jump...but the mechanics of the game allowed for it and thus the player (using game mechanics knowledge) had the character commit the act.
In last night's Shadowrun game I ran into the same situation. I had set up a meet between the PCs and their adversary. It was meant to be a high tension role-playing scene where the adversary tells them to back out of their current job and the PCs get to get all bravado/macho and say 'no'. The adversary had plenty of back up on the scene. At the end, the adversary got frustrated and made to leave the meeting...and a single PC attacked. In game, and following the narrative, this was a highly foolish move. Outnumbered no one would do such a thing in the "real world"...but the player had calculated the odds and figured that the game mechanics would favor his action.
Again a player let game mechanics make a character decision.
Now, I know that players using game mechanics in-character happens all the time. It has been this way since the beginning. In early D&D, encounters were written up without heed of what the capabilities of the characters were. It was assumed the players would make a judgement call ("are we tough enough to handle that troll?") as to whether or not their characters would engage the encounter. In fact, the "smart" players knew when to attack and when to withdraw...but it was all based on knowledge of game mechanics, not on what was actually happening in-game. This disconnect has been ongoing for decades.
Is it even possible to disengage character decision making from player knowledge of game mechanics? If we try to separate the two, campaigns will end quickly - the first time a group runs into something beyond their capabilities, but the narrative indicates they should engage the enemy, it will be a party wipe and the end of that game.
I guess there is a fine line between using game mechanic knowledge and making in-character decisions.
"The troll is too high level for us, lets leave" works for me.
"Let's kill the town guard because they are lower level than us" does not.
I know its a fine line, but I'm not sure it is possible to completely negate game mechanics/narrative decision making as much as I would to.