"AMO standards must mean squat."
AMO only means anything if the bowyer followed AMO specs. AMO says the bow should be marked 3" longer than the "string master" (steel cable) that puts it at proper brace.
Some bows are marked according to AMO, some are not (obviously). Besides that, you may like a particular bow braced at 6.75", I like it at 7.5"--that could mean near an inch difference in string length.
My guess is your first bow was marked wrong. Take a flexible tape measure, like a tailor's tape. Start at the top of the string groove, follow the contour of the limb, through the center of the riser, follow the contour of the other limb, to the top of the other string groove. That should give you the AMO bow length, or very close. If you don't have a flexible tape measure, use a string then lay the string out straight and measure it.
When possible, I take string orders by actual string length--measure the string that is on the bow that fits/braces properly, then order that length. That is the most accurate way I know of.
Chad