Just read this tip in Self and it makes complete sense:

Get a grip Stop hair from mussing up: Choose bobby pins that are flat. Pins with ridges leave space for fine strands to fall, says Kevin Mancuso, of New York city, creative director for Nexxus. (12/08. pg 74)

I love my bobby pins like no other: I have them stashed all around my apartment, in my wallet, in my bags, in my car, in my pockets - I just never know when I’ll want to pin back my hair and which way I’ll want to pin it. Up, down, half-up, half-down - my bobby pins have taken me through many a bad hair day and created some pretty good ones. I’m going to restock with some flat ones and save my ridged ones for do’s that actually benefit from a sexy strand or two falling down.