Im running a Shelly 1PM with temp add on for the hot water cylinder and Im curious about the thermal properties of water in the tank.

The setup is for a fairly modern 240l tank, 3kw lower element (actual 2.5kw - 2.7kw), shelly controlling power on / off, tank thermostat controlling the temp, shelly monitoring the (external) temps with 2 DS18B20 sensors. Both inserted against the tank, under the insulation at the element level and at about 1m up tank (where a second element would be)

As per the graph, the upper cylinder temp drops more than the lower cylinder temps when the hot water is used.  (They give the same readings if swapped - ie they are true either way)
Over time (when fully heated) , the upper cyl temp starts higher and then loses temp faster than the upper

Im just curious why this is? Water at top of tank is cooler? Heat doesnt rise? :)
Just trying to picture what magic is happening to understand the logic