I have created three closed Bezier curves. The first, "loop" and the second "shape" are simple Bezier circle that have been scaled and the scale applied. The third, "knot", started out as a Bezier circle. In edit mode, I added 3 more Bezier circles, positioned them, and then edited them to be connected into a single curve.
Next, I set the Bevel object on loop and knot to shape. I expected two closed tubes of the same diameter, but instead the diameters are different. Why are the diameters different?