I've seen a lot of people stating it worked well with other MBPs but as far as the rMBP I've seen 3 people (including myself) complain about the size and only 1 that said it worked. The guy that got Colcasac to send him a new one and pay for return shipping had to complain on the forum before he got any action out of the company. I'll be contacting them later today to see if I can get a replacement. They have a 60 day guarantee, which should help put any fears to rest. I think you may be right, there may be a manufacturing defect or there was probably an early batch made up that was off on the size.
Also, it isn't cotton. It is a polyester fleece lining. Which causes me to question the claim about allowing your MacBook to cool. It may be breathable, but typically polyester fleece is used in clothing to keep heat in. Perhaps the way they have used it does allow it to cool. Really cannot say.