I agree with you as there are no moving parts on a Phase One (other brands do have fans for cooling) , but with a high shot count that means that it as been used quite a bit.
Which could include physical handling of the back by taking it on & off camera, wear and tear on the components such as firewire ports, buttons, flash sync , cf card ejector, etc.
Thats why when purchasing a used digital back, any brand it is highly recommended it come with some sort of warranty and or someone to stand behind the purchase.
I personally have seen backs with capture counts in the high six digits and they perform great.
Lance