Well, I don't know...
I think most people don't have adequate nutrition to grow their hair at a maximum rate on their own. That's why some topical products that stimulate hair follicles actually DO cause growth. It's not that they cause your hair to grow above and beyond anything it could do before, it's just that you weren't already at your maximum rate and benefited from some outside help.
Hair growth is at the bottom of the list when it comes to important bodily processes, that's why when people get certain types of illnesses, their hair will fall out. Your body needs the nutrients it would be using to grow hair for more VITAL things when you are sick/malnourished. Your body just knows how to keep its priorities straight
While most people I think are healthy enough to feel fine, they aren't necessarily at their HEALTHIEST, so they really may not be growing hair at their maximum rate. That's when taking vitamins and doing things to stimulate the scalp will give results.
And also, like it was mentioned, your hair can be growing 2 inches a month, but if you're breaking it off just as fast, you won't see any net growth. So if you start using a different technique or something that ultimately results in less breakage, it will seem that your hair is growing faster, even if your actual growth RATE hasn't changed