Most the girls I know with straight hair that is prone to frizz are actually probably 2s. I can blow out my hair to look like it's straight--but it's not. Damp air, and out comes the frizz, and if it's damp enough, curl will start showing up.
I never realized my hair was that wavy until I had a stylist ask me if I ever scrunched my hair to bring out the waves.