I often catch myself pulling one side(particularly the top) when doing curves. It looks just like the cushions your post, mortizx. When mine does that, and one side comes up a bit short after redoing it, I note the shortage and try to "stretch" the short side just a bit in the long straights to compensate for that shortage. if its a big difference, the pattern may be off.
*edited OFF TOPIC QUESTION, I meant to ask Dan, what brand/pattern vinyl is that you used in the example above?