I have also been very happy with Karst Sports, though I haven't ordered rope from them. If it's general purpose recreational use rope I wouldn't get too uptight over 5%. With fixed rigging, when replacing a rope where I know the precise length of the existing rope, I generally add 5% to allow for unusual shrinkage, variation in rigging, etc., but I wouldn't want to use up all of that 5% in a measurement error. Kelly is right that they are most likely using a rope counter:
http://www.amazon.com/Wellington-BRM25-Rope-Measurer/dp/B000G9TXWG/ref=sr_1_1?ie=UTF8&qid=1363029274&sr=8-1&keywords=rope+counterMy local retailer uses these for rope, cord, and even webbing. I've always been a little suspicious of their accuracy. If these are in widespread use, and given that cut ropes are often used for replacement rigging where length is important, then I think I'd consider anything worse than 1% error to be unacceptable. At least one reviewer gave it 1-star due to accuracy concerns.