That a monster turns up is "diegetic".
That the GM rolls every <insert period here> to see if a monster turns up doesn't seem very diegetic. It seems like a clock designed to drive gameplay.
That the GM makes additional rolls to see if a monster turns up if the players have their PCs do noisy things (like, say, bash down doors) is gameplay with a diegetic lampshade that is vulnerable to breakage. Eg if the players come up with a solve for the diegetic element (eg the noise of bashing down a door) but still get the gameplay benefit of their action, the GM has to either ignore the diegetic action and make the additional roll anyway; or has to honour the in-fiction solve and allow the players to potentially break the game.
Versions of that last point - across a huge range of issues, not just encounter tables but almost all areas of game play, especially where spells are involved (and game-balance aspects like range, duration etc have some diegetic lampshades hung on them) - are a recurring problem with an ultra-sim game like Rolemaster. I report this from years of experience. (In Champions, @AbdulAlhazred has talked about a parallel example of the wizard character who is built so that all of their abilities are in their staff.)
