Leave it to me to complicate a simple weapon.
It's not you--it's the stores, the suppliers, in some cases the bowyers, AMO/ATA, and even fellow archers.
In most cases the stores, especially the big ones, don't know squat about traditional bows. The suppliers often don't know much either, and/or try to supply a "one size fits all" kind of string. The string is to be measured under tension also.
Several years ago I watched a "custom" string builder at a big "hunting" store make an endless string for a recurve--pulling less than 50#, if I recall correctly--he made it 18 strands (dacron)!!
Some bowyers can goof you up because many don't follow AMO/ATA guidelines for marking their bows. AMO/ATA says the bow should be marked 3" longer than the string that fits it properly (paraphrasing here), and that strings should be ordered by the AMO bow length. AMO/ATA doesn't allow for any exceptions.
Then you have many of your fellow archers that quote the old "rule of thumb"--4" shorter for recurves, 3" shorter for longbows.
IMO, the ONLY correct way to order a string--and assure you are getting the proper string--is to order it by the actual string length.
Chad