Simple Past versus Present Perfect

I was checking an essay correction and, when replying to it, came up with a grammar question. Should I say "I understood your corrections" or "I've understood your corrections"? Are them both possible, depending on my emphacizing the action in the past, or the result of that action?

Both are possible, but there is a difference in nuance which I'm unable to explain (hopefully an English teacher will comment). It depends on content, but "I've understood your corrections" is far more common.