Even during the decades after the 70s, when the front loader automatic machines took off almost completely in the US in favor of top loader agitators that dominated the american market you didn't have a standard in the US, every maker made machines in different sizes and capacities, every makes had something different.
You may say that they could've made a chart on machines sizes, but again not all machines were the same there was not even a parificated standard...maytag's biggest model could've hold 10kg, while Ge's 12kg, whirlpool 11kg..etc..
Which was a large machine? Which one regular size? Which one small? What about portables? Etc.
Answer was in an average estimation.
European front loaders with a few exceptions had standard 5kg drums and were about all the same size, so were outter tubs, they all washed with about same water quantity...exception came when automatic regulation machines came along.. infact they all advised to use less detergent than suggested....later becamse the norm and so detergents did adapt.
But there was a standard in Europe.
In the USA not...
Forget already the big difference among solid tub and perforated basket machines, but to make an example it's very well noticeable how much less detergent I need to use in my belt drive whirlpool or Speed Queen,when in the Filter-flo I need more...that lies in simple technical aspects of the machines, filter flo have a fix outter tub, only wash tub is suspended, that means a very large space between wash-tub and outter tub say 8cm....
The filter-flo fills more water quantity to wash same load size as the whirlpool, that instead have a suspended outter tub and space among wash tub of barely 2-3cm.
Load size it's the same, water level (in the tub) may appear to be the same but water quantity instead is greater in filter-flo,that calls for more detergent needed.
Detergents couldn't keep up with all these different variants.