Sue is correct in regards to the denser cooler water. Added to that the disctint lines can be different due to surface tension and wind currents too.

Depth will also effect the size of the "Boil Ring". The deeper the diffuser is placed the larger the boil ring will be. The kenetic energy created by deeper diffusers will be greater than the same diffuser at a shallower depth.

Great post and perfect pics to describe the aeration affects!