Hair starts regrowth almost immediately. Why?

I’ve been having my back waxed once a month for about four months now, using the same beautician every time. The results vary alarmingly.

The first time hurt like hell (because the hairs were so long) and I got loads of those red bumps, but I stayed smooth for about two weeks.

The second time hurt much less (hairs were much, much shorter), I got virtually no red bumps, stayed smooth for about three weeks and hair regrowth was itch-free. Was pleased.

The third time, noticeable hair regrowth took place within two or three days. And it was seriously stubbly and itchy. Was worried.

The fourth time was about four days ago, and already I can some itchy hair regrowth, though not as bad as the third time.

My questions are:

a) Why do the results vary from one time to the next?
b) If hair regrowth starts to take place almost immediately, why might that be and how can it be prevented?
c) Does holding the skin tight while the wax is applied and removed make any difference to the effectiveness of the waxing?

(Btw, great website. An absolute godsend.)

The results may vary because of the different stages of hair growth.