Consider that if they know when you wrote your review, they know which your review is, if you then change your review, that's a pretty blatant way of telling them exactly who you are, which might not go over well if they have any sway at all (or if they know someone who might be able to block your progress later).

I see. I suppose this is also a problem in small companies in which the reviews could be easily identified as well.

Some folks did, but most people by the time they are out of the door don't care.