Niklas;
I have wondered many times about that 2½"/65mm my self, since is as you pointed out not an exact conversion. On page 254 of Greener's 9th edition he shows a drawing of "The 12-Bore Cartridge Chamber". Along with dias ranging from .813" down to .798" at approx ½" increments I suppose he shows the length as "2½" or 2 9/16" Long" + an .074" rim c'bore. This would give an overall depth of from 2.574" to 2.636" (65.4mm/2 9/16") to (67mm/2 5/8").

Miller/TN
I Didn't Say Everything I Said, Yogi Berra