Heat rises so its logical for that to happen. In a heated room the ceiling will always be warmer then the floor, with differing levels of heat stratification right through the air column.
There is two heat cords in this rack one does the lower 4 shelves and one does the upper. If its a concern (1-2 degrees difference) you could put them on seperate thermostats and have the upper one turned back a bit. Or put the top end snakes up high and diamonds down the bottom. There is no rules here, the rack should be tested and used accordingly to suit what is being kept and the environment (room, shed, garage etc) it is being kept in. It is low wattage for a reason, in a controlled room it is easy to get the right temps without the problem of cooked animals if a thermostat dies. Having an insulated hide over the heat, stores and builds up heat as well