You're pretty much spot on when you say it's all about diet. In general, exercise will determine how much of you is fat and how much is lean goodness, and diet will determine how much you weigh.
For the ranges though, 1-5 is mostly for strength, because it's mostly training the central nervous system. Note I said mostly, because you'll still make muscle gains... if you're eating at a surplus. 6-7 is a weird middle ground, 8-12 is for hypertrophy or "bulking" (sorry ladies, you can't really bulk. 99% of you don't have the hormones for it), and more than 12 is for endurance, which does NOT mean "toned".
@Gnomekicker Unfortunately most of that is myth. The mass you put on isn't as strong because you haven't trained your CNS. You actually DROP reps to become stronger. Builders do many things to break through plateaus, which includes mixing up workout, but not for the reason you stated. It's actually because your body will stop adapting unless you mix things up.