Whilst I agree with you in the main, 'slightly underload' for 90% of modern front load machines where capacity is shown by weight (that is most of the world outside of nth America) this would mean the drum would still be 'full' in most consumers minds.
Interestingly, British consumer magazine 'Which?' test at 80% of claimed capacity which is probably more representative of how most use them and also helps take account of stupid capacity claims. On the other hand, our consumer magazine 'Choice', test at full capacity AND on the coldest setting...and apply a penalty to scores where cold isn't the minimum temperature!
Choice have found that to fill to claimed capacity often means forcing the last items in....not good for wash performance, possibly ok for fabric wear as it's hard to create wear if there is no movement! Famously, one American brands capacity claim some years ago was so grossly overstated that the lid on the machine was held open at a 30 degree angle with washing spewing out....
As a result, slightly under filling compared to the stated capacity will still mean a full drum - just not stuffed!