Carbon-12 is the only element with a unified atomic mass unit of an exact number - 12.000000. Therefore on this scale, 1/12th of carbon-12's mass is exactly 1. So when you're working out Ar, and multiply by 1/12th of C-12, you multiply by exactly 1. It's really hard to explain sorry haha, but the point is you need an exact number of 1, and you can only get that by using the 1/12th of the 12.000amu of carbon.