The tire size posted on the manufacture's website is probably the spec size. For example whenever they made the tire initially, they probably measured it and that's waht they use as a guide when making more and posted up that size on the website (i.e. 31.7). How they measure that I'm not sure.

I need to measure my tires.

IMO, the best way to measure the actual tire size, is to take the tire off of your jeep, lay on one side on the ground, and place 2 framing squares on opposite ends of tire, and measure the distance between the 2. This should help eliminate room for error when measuring. But then again, that's a lot of work.