Your looking at it from the point of view of a finished piece, not the tool maker...

The figures are pantographed down, now assuming that a exact height is chosen for the master (say 6" for a scale height of 5' 9") and the rifle modelled with the figure is in proportion with this... and the master is reduced in size to its scale height in 1/72 exactly (24.3mm)... then your correct...
But you cannot adjust the length of the weapon seperatly from the figure and the length will depend on whether or not the sculptur has got it in proportion to the original scale of the master.... and the tool maker reduces the master accordingly to a scale height of 5' 9" in 1/72(24mm approx)....

just about anything can cause a variation...if the toolmaker produces a figure of 23mm or 25mm which is still well within the human range, the length of the weapon will be noticably different, though it will be more noticable on longer weapons, if the sculptur makes the weapon to short or too long etc even if the figure is at the correct scale height the weapon will be wrong.... now heres the rub the toolmaker has no knowledge about or any idea of the length of any given weapon....

Now spread these people around the planet and give them limited communication...

Different perspectives here, everyone is entitled to have his own, this is mine:

I like my figures in 23 mm, Asians smaller.

I won't buy any 26+ mm figures, not matter how well they are sculpted and how dearly I "need" them, because they are not compatible to my collection. Regards, Pat

I would buy at least one 28mm figure to go with my average joe 1/72 figures and that would be a correctly done character figure of Harald Hardrada(at least according to the Saga)in Emma... and thats being conservative..

A few Figures(pun intended)
BEF would have to be 23.6mm minimum to be accurate for 1914(5ft 7inches tall otherwise you get sent away)
Roman Soldiers official height is 6 Roman feet(5ft 10inches) so 24.7mm in 1/72... but shorter(and taller) men were accepted... largest foot size I know off is size 14uk 15us for a roman military boot.... belonging to a Centurion

I have not bothered to read the expansive list of responses to this post. However, that said, I have always enjoyed the discrepancies of size believing they add a touch more realism to the craft. Many is the change of command and retirement ceremony, I participated in, where troops lined up from tallest to smallest behind the squad leaders to form the platoon. People are different sizes and piecing these slightly different scales together merely reflects that.

To quote from BLACK ADDER GOES FORTH, "King and Country would have us believe that all British soldiers are strapping 6 footers with muscles the size of Bormoth." (my apologies to those across the pond if misspelled) And the Germans never obtained divisions of their 6 foot, blond haired, blue eyed, ideal soldiers.