Gary, it about the water flow through the heat pump and control of the water in the storage cylinders.
On a one pass system (where water leaving the heat pump at a very high temp) then you are correct, however very high outlet temps mean permanent high discharge pressures, and lower efficiency. How ever if you for a full circulation system, where the heat pump only increase the temp of the flow by a few degree's per pass, the discharge pressure are kept low for longer periods. (only at the end is the discharge pressure high) for this to cylinder heat ex-changers must always bee in the coldest part of the system (I do not use internal heat ex-changers, I pump from the cylinder through a heat ex-changer and back again returned at low level)
It is all down to flow, heat transfer and available duties. You have to look at the whole application and the influences of each part of the system. What is installed is a boiler system!