Circumference vs Diameter in Hottie Xtra, Randy VixSkin, Randy Silicone, and T-Rex

So, excited by the new arrival of the Tantus T-Rex, I put together a comparison of it with the other three dildos in its "short and stout" class.

link

And I see that T-Rex's listed diameter is .25" and .125" smaller than Xtra and the Randies, but the listed circumference is .25" larger than the listed circumferences of all three.

Anybody got any insight? I'm home for the holidays away from my Xtra, but when I get home I'll take detailed measurements for this thread.