Truly, piercers tend to know shit all about stretching. Skipping sizes when they're small is a pretty bad idea, because the diameter change might not be much, but the circumference change is rather large. For example, the circumference of an 18ga earring is approx. 0.785mm, the circumference of a 14ga earring is 2.001mm, and that is quite a big jump for such a small and non-stretchy hole to make, which is why there's 16ga, which is 1.131mm. A much more reasonable jump if you ask me.
So if you look at it in terms of circumference, we're looking at
18ga = 0.785mm
16ga = 1.131mm
14ga = 2.001mm
12ga = 3.142mm
10ga = 4.524mm
Considering it's all relative and goes by logical percentage increases, it really is no surprise why the sizes don't change much in diameter in the smaller gauges. It may seem like it does no harm, but you'll probably regret it if you go bigger, or to "the point of no return", because skipping small sizes can cause seriously ugly looking naked lobes.
So yeah. Piercers don't know a whole bunch about stretching.