Why the valence shell configuration of Lead is 6s2 6p2?
This is convention. To be perfectly honest, this order is not always kept up. But it makes sense to think of the orbitals by shell (first 1, then 2, …) and not by energy level.

For lead and all elements of the p block, whether the d and f orbitals are fully populated or not does not matter, they do not take part in chemistry (only the s and p orbitals of the outermost shell do). Considering that, we can write the d and f orbitals right at the beginning where it is ‘clear’ they are not needed.

For elements of the d block, we actually are doing chemistry with the d electrons. So it is maybe more customary to actually put 3d behind 4s, just to state that they are important.

There is no physical meaning behind the ordering in either example.