The hair appears thinner because the new growth doesn’t appear right away. The hair waxed in June is not the hair that is regrown and waxed in July. It takes about 3 waxes to cycle through “all” the hair and start over again.
This is also why the first “virgin” wax is so much more uncomfortable than subsequent waxes – more hairs are being pulled at one time! As waxing continues month after month, only about a third of the hairs are pulled at any given time.
Also I think that repeated waxing causes the hair to “loosen up” a bit in the follicle, making it easier to remove.