You need me to get into the geometry? I can prove it to you mathematically, however judging by your responses so far, I doubt that you would be receptive. Fact is that the system ONLY works when the track approach angle, return angle, front arm angle, and back arm angle are almost the same. If the arms are much steeper than the track, then instead of parallelograms, you have TRIANGLES. Parallelograms retain the same circumference when you change the angles. Triangles do NOT.
If you want to prove it to yourself, draw this;
Two straight lines each 2" long forming a 90 degree angle. Measure the hypotenuse (hint: it'll be 2.83"). That means that the triangle has a circumference of 6.83".
Now draw another two lines, but make the angle 45 degrees. When you measure the length of the line connecting the two free ends, you'll find that it is only 1.53". That means that the new circumference is 5.53" and that the circumference changed by 1.3". You going to argue with that?
As for your sleds, it did NOT work for you. The fact that you only went 2.5" means that you could manage it just by running your track a little loose. When you start going to 11.5" (relocation 4" + extension 7.5"), which is common for kits they sell, then you have the track trying to stretch out by OVER FOUR INCHES!! Not going to happen!