Capt claret- perhaps you do not check your altimeters in cruise because the ops manual say so, then the comp monitor advise you only when the limit is passed, if you check it yourself you might be able to see the trend and react before you have to leave RVSM...
Altimeters are checked on the bay, passing transition and again levelling in the cruise.
ALT can be made to display by a 1hPa split between altimeters, though I can't remember what altitude split sets it off.