Over the years, I've noticed that alot of heaters of different makes don't actually heat the water to the temperature set on it but they still remain constant.
I have one heater that I have to set to 24Â°C to get an actual temp of 26Â°C (coincidentally also a Hagen Elite!), another Jager one shows 1.5Â°C difference.
The important thing is for the heater to keep a constant temperature.
So as long as you know you have to set the heater reading to 3Â°C lower than the actual temp you want, there shouldn't really be a problem. Once it's set, that's it.
I also use two thermometers in each tank ( a digital and a standard liquid one.)