The amount of PS3's don't have units of "year". The conversion looks more like this:
The above is more of an explanation than the standard method of cancelling units, but I hope it's clear now. The number of units you begin with doesn't have the qualifying unit of years, it is just a number or a number of something like PS3's, etc. The denominator does have a unit of years, thus dividing by 12 or multiplying by 1/12 converts this to months.