question on weapon req

this may be a noob question, but i'll ask it anyway.

if you have two swords,
dmg 15-22 req. 9
dmg 15-22 req. 10

but you have 12 swordsmanship
does it make a difference which one you use?
why and how?

No difference at all.

then why is req. 9 swords sold for so much more?

Because if you don't meet the requirements the sword will be treated like a 2-3 sword. The first one allows you to make a build with 9 swordmanship in it and still do some dmg. Imo it's complete nonsense because any half decent build has 12+weapon mastry, in many cases 16.