Mickey Thompson tire size chart

[ROMM]

Hey fellas, I have been wanting to buy a set of drag radials for my back tires. The Mickey Thompsons have been getting good comments on this board. I have seen alot of guys going to a bigger rim to get the 325 size tires on the back. Well on the tire size chart on the website it says that the P325/50R15 measured on a 10 inch rim has a tread width of 11.6.
For the P315/60R15 measured on a 9.5 inch rim it says the measured tread width is 11.9!!!
Can someone explain this to me? Why the heck does a wider tire on a wider rim not have MORE tread width? Also, what if you went with a 10.5 inch rim with the 325 size tire. Would that give you any more tread width? Why is a 10 inch rim the measuring rim size anyway? Is it because that is the optimal size rim for that tire? And if a bigger rim would give you more tread width, why wouldn't they use a bigger rim size to measure with on their advertising chart?

According to this chart, I can save money by not buying a new set of bigger rims and even get more tread width! WTF?