Sounds like you are ordering them by your bow length. Actual AMO is 3" shorter than the bow, under 100# tension, but that can vary quite a bit with the bow. Old "rule of thumb" is 3" shorter for a longbow, 4" shorter for a recurve, but that isn't always accurate either.
If the string is a little too long, generally you'll have increased noise, hand shock, and arm slap. Too short, you loose some performance. Way too short (or way too long), and you can damage the bow.
Too much twist in the string can cost you performance, and can damage the string.
The best thing to do, IMO, is order a pre-stretched string at the actual string length you need.
Chad