Mickey Thompson tire size chart

[Silverback51]

I can provide two possible explainations.

First, it may be a misprint.

Second is that the 315 and 325 designations represent the cross section width of the tire, not the tread width. Depending on the tire construction it is possible for as smaller size tire to have a greater tread width.

It's not unusual to see different makes of tires for the same size to vary in tread width. Pirelli tires are quite often amoung the widest as they tend to be a square shoulder design.

Hope that helps.