I wonder too if the 16" doesn't have something to do with the walls
being able to take the weight.  Those of us who live in the northern
climates get a lot of snow on our roofs in the winter.  Spring snow,
like we've had the last few days, is wet and heavy.  Like someone
said, while 12" would give more strength and 24" less strength, 16"
seems like a good compromise.  Just my 2 cents.

Andrew