At a guess, DM's houserule.
And in my opinion by first glance a bad one. If a person can reach level 10 by making swords for 4 years something is off.
And how do you become a better fighter, wizard etc. by making swords?
Are you taking into account that they'd be doing things other than making swords with their time?
I used the standard craft rules (poor as they are) and the weekly work progress from them. And as I rounded up it should leave a couples of weeks for doing other stuff per year. I didn't model modern times and vacation, in those days your worked almost everyday unless you were rich.