Mauser is to blame, because although being the originator, the company did not use the case length in naming it. The cartridge is from 1889, while the “metric” naming scheme (Erfurt convention) started in 1909.
As a matter of fact, as late as October 1939 the Mauser company, called this cartridge “7.65 türk.” [Turkish]. This is surprising to us today, but from an Oberndorf view, Turkey was the dominating export customer. Neither Belgium, which produced the rifles itself at FN, nor Argentine, which was served by the Loewe rifle factory in Berlin, were as important to the people at Oberndorf as Turkey was.
Commercially, the cartridge was practically non-existent in Germany. For a short period time after making licenses for buying ammunition mandatory, you could nevetheless buy 7.65 Mauser cartridges, because this ammunition type was not officially listed. The incoming flood of Argentine surplus ammunition soon changed this.
The “correct” case length (53.3, 53.6 and 53.7 mm can be found in factory drawings) does not really help either.
CIP has chosen 7.65 x 53, coming from Germany and Belgium.
In Argentina it is called 7.65 x 54 if my notes are correct.
Calling it “7.65 Mauser” would be unambiguous and avoid the case length confusion.