the short answer is sensors aren't too accurate. I mean they are not calibrated to anything. You'd have to connect them both at the same point in the loop and see how far off they are from each other, then correct on aquasuite with an offset on one of them.

That said it doesn't seem abnormal to me. It's mostly the GPU that heats up the water a lot in normal use. Because you have a rad in between the two blocks, you don't really know what temperature water the CPU block is getting. And your pump speed will make that difference vary a lot.
If your pump runs at full speed a 420mm rad may decrease the water temp by 0.5°C, if you slow it down maybe 3 or 4°C. It's very variable.

I never concern myself with absolute accuracy, assuming the error to be fairly constant.
I am more interested in change.
My typical loop compares coolant temperature to ambient air temperature, increasing fan speed as that delta increases.

The temp sensors in computer cooling loops are just cheap 10K NTC Thermistors which typically have an accuracy of +/- 5% in the temperature range encountered in a computer. Aquasuite allows you to add an offset to a temp sensor so you can calibrate them by sticking them in a cup of ice water and adjusting the offsets until they both read 0°C. Their accuracy is not linear over the entire range of temperatures they can measure but accuracy is basically the same over the range of temperatures they will measure in a computer. They are generally more accurate at lower temps (like below 50°C) because their resistance changes more per degree C at lower temps. As the temp goes up, the resistance changes less so its harder to measure accurately. Here is a resistance vs temp graph for a typical 10K NTC Thermistor.



I have 3 of the #53206 temp sensors plugged into a Farbwerk360 and coiled up in the back of my case. The temp they report varies a bit, normally a few tenths of a degree C. If you want higher accuracy, Aquacomputer still sells the Calitemp sensors but they report via Aquabus so they can only be used with an Aquaero. They also cost about $20 each. I don't know what type of sensor is in the Calitemp sensor. Thermistors can be Class A or Class B. Class A Thermistor accuracy is +/- 0.15°C @ 0°C, Class B Thermistor accuracy is +/-0.30°C @ 0°C, so the Class A Thermistor is twice as accurate as a Class B Thermistor at 0°C. 10K NTC thermistors are Class B, which are pretty accurate but not as accurate as a Class A Thermistor.