> 79.3% of pours were short (under 100% of claimed volume)

That is quite surprising. What breweries used to do when I worked in cafes and bars was to paint a mark on the glass and under it write "0.5" (in market that sells in liters and not in pints) and this mark is above[1] the half liter mark. Note that it is without units.

Pubs and bars get glasses for cheap or free and the brewery/distributor tries to trick the bar into selling more beer than they expect.

[1] We checked using measure cups from the kitchen. Maybe there is a conspiracy to have kitchens use too little ingredients.