Why do hairs grow at different rates?

The reason is that it is genetically programmed into us while we are developing inside our parents. As little babies your body develops as a series of segments and each of those segments inherits its own genetic programme. In some areas the length or growth phase of the hair follicle is longer than in other areas. In your eyebrows for example, a hair grows for about a month and on the top of your head a hair follicle is active for anything up to three years. Hair follicles have three phases to their life cycle: they have a growth phase, and this is called the anagen and can last up to three years for a head hair, 21 days for an eyelash and 3 weeks for a pubic hair. If you go through the growth phase, the next phase is what's called a catagen phase, which is when the hair falls out, and then you have a resting or telogen phase, and then the cycle resets itself. The length of hair depends on the length of the growth phase.