Fluval Heater

I have a Fluval heater in my flex 123 but when its in the back compartment its impossible to see the LED readout so I put my digital one in tank aswell.
When I filled tank I ramped up the heater to hopefully get it to temp quicker.
Well today the digital read 24.5 so I thought il turn down the fluval to 24 from the 27 I had it set two.
Anyway when I went to do this I pulled it up slightly to see what I was setting it at and water temp was 27 ?.
So turned it down to 24 and it then started to flash red as it should.
My question  are the digital so inaccurate it could show a 3° swing ?

I had my heater in tank and a digital aswell and the digital was showing 3° or so cooler.
I took that out and put it in my other tank and same thing 3° cooler than that heater. I then put both heaters in same tank and they read the same temp so presuming they are correct and the digital has lost its calibration. Its in the bin

Heaters were fine  both Fluval heaters same reading it was my Digital heater that was 3° out