The "catch" is a lot of bows/bowyers don't follow AMO specs.
Some (actually most, at least in my experience) are not marked according to AMO--which is, as stated, 3" longer than the string length. That's the "rule", but consider this--you have the same model and length bow, but one guy likes it braced at 6.5", the next likes it braced at 8"--so what's the AMO length for that bow? Depends on who is shooting it.
I've seen bows get strings anywhere from 1.5" to 6" shorter than the marked bow length. Then you have bows that aren't marked (selfbows, ink worn off, refinished, etc.).
The correct way to measure a bow without a string is start at the "top" of the string groove, follow the contour of the limbs, through the center of the riser, to the "top" of the other string groove. You'll need a flexible tape, like a steamstress' tape, to do this. You can do it with a bow strung or unstrung--it will measure the same.
Chad