Why do people correct the same writing again and again?
Yes, I agree. Sometimes people make corrections of a previous 'correction'.
I don't teach here, so I don't do many corrections. However, I occasionally browse the Notebook pages and see instances where a person's writing has been 'corrected' incorrectly. In some cases, the person who has supposedly corrected an entry has actually introduced errors that weren't there before. I have often seen situations where the original poster has written something right, and someone else has come along and changed it to something wrong. If that happens, it seems only fair to step in and offer a 're-correction'.
It also depends who made the earlier correction. If a Chinese, Turkish or Mexican member, for example, wants to have a go at spotting the mistakes in another learner's English, then that's fair enough. In these situations, one can presume that the person who posted the entry will realise that the correction may not be perfect, or that the corrector may have missed something.
However, when a native speaker has made a wrong correction, then you do feel duty bound to set this right, and all the more so when this person is registered here as a teacher. Unfortunately, this happens more than you think.
I do it because sometimes native Spanish speakers just add new mistakes to the entries... like spelling mistakes, they cross out words that they don't know but that do exist, grammar mistakes, they introduce words that are unique of their dialect and write "this is the standard way to say so", they don't type accent marks and so on. And also because I prefer to write what sounds more natural to my ears and not the direct translations from English, French or German. Sometimes, people who correct don't get the real idea about what the learner wrote and changed the whole idea, sometimes writing things that don't make sense in Spanish. Most of the time people want to be helpful by correcting entries, in some cases those corrections are not even useful at all, but it depends on the learner how he or she takes advantage of the corrections.
Maybe because there are several ways to explain the same things...
In my case, if I correct something that's already been corrected, it's usually because I noticed errors that the other person missed.
That's what I meant, that some people feel the need to re-correct all the time, although the writing is well corrected. Besides, if your writing has been corrected seven times you don't know what is ok.