the diameter of the tank, coupled with the shape of the funnel and i guess other design elements cause the liquid inside the tank to not travel to all areas of the tank equally. the i see that it is not consistent has to do with the fact that as you described even in a perfectly level machine or perfectly level tank the whole thing flexes under pressure. depending on the amount of chems, amount of films and even temperature, i think would affect the over all results and exasperate or reduce the density difference.