I get that. It's the power of the brain's user of generalizations, i.e., patterns or classes, to represent things. The brain clearly also handles disambiguation far better than we consciously know to do. It seems like the brain essentially has a class named quatre vingt and it has a pattern of 4*20 that resolved to the concept of 80 which means 20+20+20+20.
It clearly comes from a lack of having a separate term for 80 or even 90 for that matter the way that German and English do; which I find peculiar too, considering that French a Romance language (not the heart romance), while the people are largely Germanic in origin, i.e., the Franks. It makes sense when you consider how the roman numeral system functions and that the Franks were in far closer proximity to Rome than the Germans, including the ones that moved to the British isles and became the English, i.e., Anglos and the Saxons, Germans. It seems that those interplays and intersections with the cultures are what determined how French language numbering worked based on when and where and what they had contact with.
It clearly comes from a lack of having a separate term for 80 or even 90 for that matter the way that German and English do; which I find peculiar too, considering that French a Romance language (not the heart romance), while the people are largely Germanic in origin, i.e., the Franks. It makes sense when you consider how the roman numeral system functions and that the Franks were in far closer proximity to Rome than the Germans, including the ones that moved to the British isles and became the English, i.e., Anglos and the Saxons, Germans. It seems that those interplays and intersections with the cultures are what determined how French language numbering worked based on when and where and what they had contact with.