It would seem that there wasn't a lot of control over the weight of the centers, I have weighed several and find most of the early centers weigh in at .590 to .60, these would be centers from strikes that had the .999 on the rim and .999 on the silver it self, I have only weighed the center of one strike made in 2005 and found it to be .50, so I guess it will all come out when they start gpoing to melt. The only place it will make a difference is if people are buying just for the silver weight and then I guess you have to base your buy price using an average of say .550 just in case there are more at .50 than .60
I haven't really paid much attention to the weights by Mint maybe there is where the difference is.
Walt