I was looking at what grants a player corruption and I noticed that a player gains the same corruption for doing something that pleases their god (ie khorn, beating a superior number of equal strength foes) as for failing them (ie khorn, being defeated one to one in combat).

I understand the failing, but the reward seems like something that should be avoided, particularly if their getting close to their corruption limit.

Am I missing something or not quite understanding something?

Pleasing a relevant god also gets you infamy in addition to corruption, and neither value is particularly large. That may still be problematic if you're getting close to the corruption limit, but if so then you've not been pacing yourself as well as you might have. Corruption is a bit like the life total in MtG, in that the only point that really matters is the last one.

Plus, keep in mind that if you get corruption from doing "good" things then you have the possibility of pushing the result and either rerolling (if unaligned) or potentially getting a reward from your god (if aligned). If you get it through a failing then there's no way to push things around from a crap result to a good one.

As mentioned, if you gain a Chaos Gift for being awesome, you can roll to swap it out with a Chaos Reward from your patron or, if you have no patron, roll twice and pick one to keep. If you gain a Chaos Gift for sucking, you are the mercy of the dark gods. Of the dice. This is worth restating and remembering.

Also as stated you usually get infamy for corruption gains for pleasing the gods but you do not when you gain corruption for sucking, making you that little bit closer to spawndom over godhood.

You also lose ability to modify your result on gifts table by infamy bonus if you crossed corruption treshold by failing.