I've noticed recently that sometimes when I'm messing with corruption loadouts, I'll get a particularly good one that gives me ~10% more ish, and then when I ascend in it and try it again it'll give me less despite the score being the same.
For example, in these two ascensions I used the same loadout with the same score and no upgrades purchased in between, but the second ascension's hepts/sec were lower. 
