Older Version Spawn Bug?

[ZomBrony]

From the title, this may sound like it belongs in Support, but I really don't think it does. I'm playing Tekkit on 1.6.4 and even after removing a dungeon mod, it seems that hardly any mobs are spawning. The Entity counter is constantly fluctuating and I can see them underground on the minimap, but I'm used to them spawning just as excessively on the surface in newer versions. Is this just how 1.6.4 was like I'm starting to imagine or could the lack of spawning still be caused by a mod? I'm playing on Hard mode so 1-10/<100 mobs really doesn't seem right to me.