Or maybe they are just trying to justify for their higher asking price.
I dont have the Mark V in hand just yet (hopefully I will have him in two weeks time) so I can only judge its design from the specs and other collector's in-hand pics.
From what I can see, the Mark V is constructed with less pieces of components than the Mark 1.
It is also not as equipped with as many gadgets, weapons, extras as the previous Marks (no back air flaps, leg air flaps, arm shield, extra RD Jr head, arm missles, shoulder missles, Retractable countermeasure dispenser, etc).
The HT Mark V seems to have a more detailed/complex outer "shell/armor" compared to Mark 2, 3, 4 and 6. But IMO the other Marks are more equipped and way more complex in terms of design/engineering than the V....But that is just from what I can see from the specs and in-hand pics.