Multi-Gauges, Units Of Measure, Tradition?

While looking to upgrade my old black aluminum perf gauge, I noticed multi-gauges with measurements in 1000s of an inch as well as mm.

I assume that until a certain point, Canadian and US stamps were designed and measured using fractions of an inch? If so, why do we measure perfs, stamp design size (flat plate, rotary press) in mm rather than 1000s of an inch? Tradition? Carry over from WW stamps?

I'm just curious. Philatelists are very exact about most things. Why hasn't the hobby converted to measurements actually used to create early US and Canadian stamps?

J A LeGrand (and P Mahe) invented the first perforation gauge in 1866 in France and he set the standard based on 2cm which was the average length of a stamp so that one could count the number of perforations on the stamp. His gauge was actually based on the perforated sides of stamps themselves from different countries and he placed his count in 2 cm besides each one. Later, some specialist started going back and researching perforating machines and found that those early ones used in the U.S., Canada, Great Britain, etc. were based on the Imperial measuring system. See http://en.wikipedia.org/wiki/Jacque...philatelist) and http://en.wikipedia.org/wiki/Pierre_Mah%C3%A9

Still seems strange to me that perf and stamp sizes for classic US, Canada, etc. aren't exactly precise. Imperial measurements are converted to metric. For example, what is the actual design size for flat plate stamps? Obviously one could convert back, but is there an agreed upon size in fractions of an inch as there is in mm? (18.5 - 19 x 22).