I looks as if that figure is based on the proportion of frontline troops.
Prolly kosher for WWII huge numbers were used for the logistics that kept the frontliners fighting. Expecially for the US and UK, that didn't suffer massive defeat at some time. (That tends to mess with the best laid plans and organisations.)
Considering how WWI was an infantry war the figures look a bit suspicous.
France lost approx 1.3 million dead out of 8 million mobilised that indicates a huge proportion in harms way at the front, way more than 20%.
OTOH when looking at French WWI photos, people only seldom carry rifles. Usually the lug ammunition, handle artillery pieces, trench mortars etc, or man machine guns. Which is consistent with the French empahasis on technology in WWI.
This makes sense. Rifle rounds accounted for no more than the odd % of the casualties anyway, so 'firing shots on the field of battle' may not have been a priority compared to firing mortars, arty, opening gas cannisters, crushing people under tanks or any of the other unpleasantness going on.
(Weirdly enough in their photos the Brits almost always carrry their heavy Enfields.)