Back in the days before digital printing became viable, printing presses (of just about any technology) would require set-up and test runs before they would produce a given job according to spec. Colors would be mixed, screens opened (or allowed to close up a little), impurities revealed and removed (you know... like a strand of hair on the print screen/plate). If someone ordered 1000 units to be printed, you'd burn though maybe 100 units just getting everything calibrated and fine tuned just at the outset. But it could be more, and it could be less. And, for a multi-color job, where registration was even more of a factor, you'd have those first few failures for every single pass through the system.
So, if a customer wants 1000 units, you'd have to start with 1000 + X in order to absorb the slop of setting up the job, but you could never be sure how much of that slop you would actually need. If you went through the whole process of producing the job, and in the end came up a little short in your total run (say, you ended up with 950 usable pieces), you didn't want to have to set up everything all over again to just catch those last few missing pieces. Almost all of the work was in the set-up. You'd be doing the job twice at that point for what was such a small percentage under run.
Likewise, if you ended up with a few extras, your customer typically wouldn't want you to be hanging on to the overruns (nor would you necessarily want to hang onto them), because the customer wanted full control over the finished product. If I order 1000 copies of an item, and tell the world, "Hey, world, there's only a thousand of these!", but in the meantime, the printing company is sitting on an extra hundred or so of the finished product, that could put me, as the customer, in a bind.
(I had that happen once when a friend of mine and I had designed some t-shirts to sell. The print house printed them on the wrong color. But when we said we didn't want to pay for them, he said, that's fine, I'll just sell them to the college kids down the street. We were screwed. We ended up buying the "bad batch" off his hands at a severe discount so as to not allow our intellectual property out of our control like that. Oh, and we never did business with that print shop again.)
So, the tradition in the printing industry became that as long as you came within 10% of the target, the printer wouldn't have to re-do the job, and the customer would pay for what was delivered.
The thing is, now digital printing is much, much less set-up intensive, and generally much more consistent, as well. Color registration and stamping/cutting are computer controlled for most processes. For shorter runs, it's much cheaper to go digital for most printing processes, although for very large runs, the older printing techniques can typically still produce more product at a faster pace, once the set-up is completed. A house like USPCC produces *so much* in high volumes, I'd be shocked if they could abandon their analog printing techniques. But, newer shops who cater to smaller orders (like, say, the Taiwan factory that services LPCC and EPCC) can go all-digital. For shorter runs, they will not only be cheaper, but substantially more precise.
As such, for the newer and smaller shops, the 10% margin of error is going away as an allowable business practice. Underruns are easily addressed; overruns are unnecessary.
This is, of course, a simplification, and accounts in general for a wide variety of differing printing techniques that have their own specific considerations. But, you get the point.
