Well, I'm just spitballing here...
I'd guess that it shows a radial density contrast between two convective regimes near the surface.
.......
Just spent 5 minutes reading my copy of "The Sun as a Star". Apparently in addition to granulation which is the visible pattern of convective cells, there is also evidence of larger scale, though smaller amplitude. convective behavior called supergranulation.
The difference in convective structure between the surface of the deeper convective zone, and the visible convection, implies a contrast in density among other things. A well observed and understood explanation for the MDI data.
Still bored?