Braces: what does it mean if my top teeth arent as crooked as the bottom?

Just that...: It means just that; upper is less crooked than lower arch. Teeth are held by gums and bones underneath, as you know. Now, the bones are remodeling everyday, every second, although adding and losing is 50/50 and they remain stable. Teeth do move however, as a slightest change occurs in mouth--e.g., cheek muscle mass, sleep pattern, chewing/breathing habits---more lower teeth get crowded with age.

Great: That much less to do. But to get best result, orthodontic Rx required in both arches to coordinate the occlusion. Call an Orthodontic Specialist for an initial exam (usually free or modest cost) for an individualized evaluation. You'll be glad you did.