There's also huge difference between bikes. Take an old Hi-Ten Schwinn frame and you can probably get away with something half a cm too small and just cinch it down. Take a higher end aluminum frame and tighten it enough to hold a seatpost .2mm too small and it'll probably crank the frame at some point. I've seen frames ruined by using .2mm too small and too large posts. I've also seen people riding around with pieces of water pipe holding up their seat. The answer that you "can't do this" is "correct", but sometimes you can get away with things. But it's up to you to decide how forgiving the parts you're working with are and how much it would suck if you ruined your frame...no single right answer to those questions.