Curved corner rectangle irregularity?

[-=Drifter=-]

I have been doing the gel button over and over again so I can do it automatically. I love the look of that button and Gary's tut is so simple and great!
I have found something odd - and it occurs both in Xtreme and Xtreme Pro.
If I simply draw the rectangle and watch the dimension numbers above and stop when I get to 300/100, and then change the curved corners to .7 using the arrows, it comes out fine. However, if I draw a rectangle close to those dimensions then type in 300 and 100 in the dimensions boxes, and then adjust the curvature to .7 using the arrows the rectangle is a different shape.
This happens consistently.
Why is this? Am I missing the obvious?