People have done some pretty creative things with BikeCAD. I would recommend opening up some of these models you've found and investigating what has been done to arrive at the design. While it may not be immediately obvious how certain tubes were created, it can help to recognize there is an unlabeled checkbox next to each tube in the tubing dialog box and also in the seat stay and chain stay menus which can turn on or off that particular tube. By systematically unchecking and rechecking each of these boxes you can determine which tube was used to model each aspect of the design. Once you've isolated a tube which has been used in an unexpected way, you can turn on and off the curved tube checkbox to see how  that feature might have been used as well.

When using a hooded dropout or any of the dropouts available in the static drawing menu, you can freely move the rear end of the seat stay using the Sx and Sy dimensions in the Dropouts dialog box. I have noticed people taking advantage of this fact to repurpose the seat stays (or the second pair of seat stays when using the Symmetric double option) to represent entirely different tubes.