When you say immersion, do you mean the solar coils which heat the water from the solar panel. If so, this is what I have, and it makes sense to me.
Remember : Hot water will rise, but will not drop.
A heating coil (of any kind : electric, gas, solar) at the bottom of the tank will heat the water at the bottom and this will rise and the temperature of the whole tank will rise.
A heater at the top of the tank will heat the water at the top of the tank only and the water at the bottom of the tank will not heat.
The solar coils at the bottom will heat the entire tank. I also have an electric and heater coils in the top half of the tank. They only heat the water above them so work faster, because there is less water to heat.