I think it´s because the hair is reduced in diameter and pigmentation.
When you regrow some hair, there is no new hair if you look closely into the roots, it´s same as alwyas. It´s only thicker hair fibers, and more pigmented.
Also thicker hair looks much better, because thinning hair (even slightly thinning hair) don´t stick together very nicely, and don´t shine like normal hair, at least in my case, but looking to thinning guys it seems to be a general rule.
The big 3 has done a good job in my top/front hair, I haven´t realized how thin it was until it has thickened. I didn´t remember my pre-male pattern baldness hair. I hope it could do the same in my vertex, which was my main concern, but i´m losing faith after 1,5y...