yep - more than a few bows labelled 60" are really 59" or 61" - all depending on how ya measure them, and that process can be a bit tricky, too.
we all need to get with the program and face reality - NONE of this bow length and bowstring stuff is exacting. the fact that strings can, and need, to be twisted sez it all - its working length will change. so will it change as the strands adjust to minor length anomalies and any inherent stretch is mitigated.
no matter what the bow length - either measured or labelled - it could easily take a shorter or longer string for that bow to achieve its best performing brace height.
so, there is NO exact bowstring length, unless this was the 70's and the string was kevlar. let's not go down that road, hah!
yes, bowstrings can change/shorten their length by twisting, and some twisting should be done to round it off, make it more aerodynamic and get those strands to lie together for maximum strength and performance.
i'll say again - a tillering string can take a buncha guess work outta what string length to use for that bow ya just bought that didn't come with any string, or length label on the riser or limbs. :D