What Eightbore said.
Imagine just a high spot in the middle of the hook. That would be enough to set the barrels back tightly against the breech face when closed, yet allow the the hook to "rock" on the pin when open. In otherwords, when fitted, it may not have been fitted with maximum contact. I suspect a similar scenario (varying degrees of original surface contact)could explain why some toplevers with rotary bolts move so quickly past center while others with many thousands of rounds through them move very little.
