Also I'd like to mention what Ian King heavily stresses, rep ranges depend on your training age!
So you have to start by determining for how long you've been training, and what's the base norm rep range for a given muscle group, and THEN up it.
However, using concepts such as "I've trained for 2 year" doesn't do it, you have to think how GOOD you've trained.
Most intermidate trainers get good results in the rep range of 6-12, beginners go 10-16
Advanced trainess often benifit in 4-6 reps.
Also, this is kinda common sense, the better you are, the more you can lift, the more you can lift the more active your neurotical pathways are.
The more active they are, less repetition is needed to fully activate them.