Pierre-Yves Gaillard and I have just answered the same question a few moments ago. We have no up- or downvotes but there are two comments to Pierre-Yves's answers.
My answer was, quite naturally, behind his since I answered later.
However when I went back to my answer, wondering if I should edit some details, I noticed that now my answer was before his. As I was intrigued, I went back a few times to check and indeed our answers seemed to be permuted rather frequently.
This is strange because neither of us edited his text, there were no new comments and, as already mentioned, nobody voted on our answers.
Needless to say, I don't care in what order our posts appear , but I'm frustrated not to have a clue on what's going on. Have I missed something obvious?
Update OK, now Pierre-Yves has an up-vote and I none, so his answer quite logically precedes mine. But I'm still interested in what was going on a few minutes ago!