This is something I've noticed in all of my writings, whether it's an essay, a diary, a fiction or anything else. I tend to start writing in a certain style, then at some point, I will realize that I switched to another one in the course of writing.
The changes happen more or less gradually but will inevitably end up in a dictinct, generally closer to my inner voice, style. Of course, the longer the piece, the more this is prone to happen. Also, I find it very hard to edit such a text afterward.
Is this something common? Is there a way to avoid it? If not, how can I make the best out of this tendency?