It's a question I've asked her service providers many times. The supervisor of our behaviorist says everyone--you, me, our crazy neighbor down the street--has good days and bad days. I am aware that we all have mood fluctuations, some days I take things in stride, other days the same things would set me off. However, this seems to go beyond that. My daughter can be so spacey and unaware on some days, like she's on another planet. These also tend to be the days that she doesn't transition well or handle her emotions well. Other days are like yesterday. She plays well, is easy-going, is social, and is totally in-the-moment and on planet Earth.
I wish science could figure out what causes these differences and then make it so all days are "on" days. That would be cool!