The ones I got that were bad were CAD drawings. It is so easy to push a wrong button, it comes out 10% off and it isn't noticeable until you cut a tube or two.

I don't know how they screw it up, but it could be the same as with printing on a printer. There if you print something full size, or fit to the page, it may not be obvious that there was a mistake. There are a lot of other ways to blow it.

There is also the whole paper shifts in the weather thing also. Which is why some prefer mylar.

If one just takes the measurements direct from the machine space it eliminates the complications, and saves on the cost of the print.

I am not saying this will happen to you every time. The time I first got caught was building guitars from LMII plans. They are a professional provider, and I was all in building forms for the bodies and such when I noticed that I had blown it. I build all those fixtures first. If I had started with the neck, which is what I do when I have the fixtures, it would have been pretty obvious the half scale was not 12 inches ish.