It must be because it acts as a heat sink or radiant heat shield to the food, causing you to open the air port more to burn the coals hotter, and thereby consuming them faster.
Here's my next question: How deep does everyone set the top grate? Can you fit a turkey in there without using a Weber lid or does that put the grill too close to the coals causing flare-ups?
I bought the 1 ft diameter weber charcoal grate to use as the fire box base, and I am looking for 1 ft tall expanded metal mesh for the sides. I am thinking I can set the grate two inches up from the bottom to create the ash-relief and get good airflow, leaving 10 inches for loading coals. Sound like a plan?