I think of 6x9 as a convenient, conventional, description, for the reasons that you give i.e. film gate sizes can vary by a few mm.
Granted that the metric system can be more convenient for scientific and engineering use, but the "Imperial" system, with variants actually in widespread use in many countries before the French Revolution, can be used to be just as 'accurate'. And it evolved for ease of human perception - think ...
Inch -Thumb -Daumen - Pouce ... ;)