How is it possible for my hair to be curly on some days and almost completely straight on others?

I don’t mean throughout periods of my life, I mean from day to day. The majority of the time it has a lot of natural wave, but there are times (like today) where it is loaded with springy curls. Other days it’s quite straight.

I really thought your hair type was meant to be a relatively steady thing. You have curly hair or you don’t. I air dry my hair the majority of the time, and I don’t use any styling products.