I have read an FAQ about this, but can't find it to link to.
They stated that it is a design feature - the thermostat always heats to above the desired temperature because the room/house will cool down. Their claim is that this minimises the number of times that the boiler and pump will have to switch on and off, so reducing wear and increasing longevity. Before tado, ours would sometimes click off then back on again 10 minutes later. The old thermostat had about a 2 degree split between off and on.
I've changed the desired temperature to lower than before the tado because we were sweltering at the peak. It has settled a bit now and only goes half a degree above, then drops to half a degree below before firing the boiler back up. I'd rather that range was narrower.