Why do fingernails curve down when growing out?

I believe it's because the keratin that makes up your nails grows at different rates - the top grows slightly faster than that on the bottom causing them to cuve.Why do fingernails curve down when growing out?
when they get LONG they curve down so you can still pick your nose.Why do fingernails curve down when growing out?
they do when they're like half a foot long but before they get to extreme lenghths they don't...they do straight out