I wonder why the lack of consistency for something so basic as telling time.
IMO, consistency wasn't really so high on the priority list in their diaries. Another example of inconsistency is that Olga would sometimes capitalize terms like "Vsenoshnaya" (a religious service) and at other times did not... Often you could tell by the way she was writing that she was in a hurry and doing it fast just to sort of get it over with, as part of her duty to write in her diary every single day. At other times, she would go into a lot more detail, her handwriting looks more careful, and she would even mention her feelings here and there - unusual for royal diaries, which normally just record facts. So it probably all depended on her mood that day and what happened that day, and what she was recording. When you are reading the actual written text of the diary as opposed to its published text, there is a lot more you can pick up on, it is a lot easier to "read between the lines", so to speak.
