what kind of grinder? If it's based on a cage spawner dungeon, those only ever spawn up to 4 mobs and they do not care about anything that relates to the mob cap or the presence of other players. If you are not getting the full 4 mobs, then either the server has done something to limit the spawn rate of cage spawners OR you still have some light leaking in that's causing some of the spawns to fail.
If it's a natural-spawning darkroom grinder, wherein the mobs just poof into existence like they do on the surface/in caves, this can be caused by having other players on the server that are in the same dimension. The mob cap does in fact account for their presence, but no such guarantee is made to make sure the mobs are evenly distributed across all players...as such, someone else may simply be getting lucky enough to have more mobs spawn around them whether that be in their own mob grinders or in the unlit caves under their base (caves can be sparser or denser in different areas).