I tried to look it up, but I couldn't find any convincing answers. I did find one answer saying it has to do with the milk's density, but the density is about 1.03 (both dairy and soy milk) and when you run the numbers you get that 936 mL is only 0.964 kg.

There is also no pre-metric units for volume, so that isn't it either. Also, some other milk brands sell their milk in 930 mL, 1857 mL, 1858 mL or some other really arbitrary number.

My guess is that it's close enough to a full liter so that the customers buy this thinking they got a liter of milk.