Waiting in line at the supermarket is a great analogy in the sense that it's not uncommon for each person to take a differing amount of time to service.  One person with an overflowing cart may take a short time to process as they have mostly bulk items and pay with credit card.  The next person seems to have just a few items but tries to pay in cash and ends up needing to put several items back since they don't have enough cash on hand.  Throw in things like price-checks, or Karen needing to see the manager, and its easy to see why progress is hard to measure in a uniform way.