It seems like less mobs spawn when there's no lighted area, but thats because mobs have more space to spawn and they are more spread out. Minecraft keeps attempting to spawn mobs until the mob cap (70) has been reached. So when you restrict some of the area where mobs can spawn (in this case lighting it up), the mobs have less space to spawn, but Minecraft still attempts to spawn mobs until the mob cap has been reached and because of that it seems like there are more mobs, but in fact the mobs are just more crammed up.
It seems like less mobs spawn when there's no lighted area, but thats because mobs have more space to spawn and they are more spread out. Minecraft keeps attempting to spawn mobs until the mob cap (70) has been reached. So when you restrict some of the area where mobs can spawn (in this case lighting it up), the mobs have less space to spawn, but Minecraft still attempts to spawn mobs until the mob cap has been reached and because of that it seems like there are more mobs, but in fact the mobs are just more crammed up.