This is because one (the X1) is tapered - you "experience" the thin side (keyboard) while the thick part is on the back.

The 450/60/70 is just a flat slab so it maintains its thickness along its whole surface (although the 460 and 70 cheat and they become thinner about 1 inch before the end, so they look a bit thinner when looked at from the front).

I have a 450 and don't mind it at all.

The T4[567]0s series are tapered at the edge as well. The -s suffix is the lighter version, basically similar to X1 series but with more ports/battery options/upgradability.