That a monster turns up is "diegetic".And in this case you are factually incorrect. Most OSR games are very upfront about the function of encounter rolls. That function is only peripherally diegetic (at best). I don't give a good goddamn how many times you repeat the same biased and incorrect things btw, but if your throat is getting sore I'll happily offer you a nice lemon lozenge.
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.)







