I think some people's hair clump differently from day to day, so when your stylist cuts your hair it's all even that day. But next time, it all clumps a little differently so you have hairs (of differing lengths) from different clumps forming a new clump.
Just a theory.
3b/c, CG 9/18/08
Last process: 04/05/08 Big Chop: 08/08/09
Currently: Herbal Essences Hello Hydration, Herbal Essences Set Me Up gel (summer only), Boots Curl Creme (winter only)