Well, as they (I) like to say, theory isn't everything.
Then we have my beat-to-death ex-school unit with consistent spring feel and only one missing rivet...
There are more variables at play than just NIB/non-NIB. The fact is that the Model M construction technique is of "questionable goodness" so NIB doesn't guarantee it'll even have a usable number of rivets left. Certainly, NIB is more likely to be in good shape than obviously abused, but the way some of you people talk, you act like it's a guarantee.
Consider that NIB can, without dishonesty of any sort, include "dropped off a third floor balcony before it was given to us to sell".