Things were indeed confusing on Brit guns prior to 1887, after which they started using the "fractional" bore sizes (13/1, etc), which gave you a much more accurate idea, when purchasing one used, how much the bore might have been honed.
But in any case, not unusual to find a difference between the gauge standard and the true bore diameter, even on much more recent guns. I had a Sauer 20, made in 1936 and imported by Abercrombie & Fitch, that had 19 bore barrels. So marked on the flats, and that's what they measured.
