It never made much sense to me. How statistically "an hour" came out, probably relates to overall survivability at the time it was loosely defined, and with improvements to care, it's now clearer how many prior "they didn't survive" might have, given more immediate care.

Richard Holmes worked from the tallies WW1 Surgeons had to complete, documenting how battlefield dressing stations and rear echelon surgery worked in those days and it was pretty awful. But there are remarkable tales of resilience and survival right the way back to Waterloo.