You made my day !
Nice custom messages =)

 DietPi v7.1.2 : 14:50 - Mon 05/03/21
 - Device model : RPi 4 Model B (aarch64)
 - CPU temp : 39'C : 102'F (Cool runnings)

Then …

 DietPi CPU Info
 Use dietpi-config to change CPU / performance options
 Architecture |     aarch64
 Temperature  |     28'C : 82'F (Who put me in the freezer!)

That’s quite singe mismatch. You ran both shortly after another? They should use the same underlying function: print_full_info=1 G_OBTAIN_CPU_TEMP

same raspberry … before and after a new fan was installed.
First “CPU temp” from the banner right after I log in.
Then “Temperature” from the cpu command.

Okay then it all makes sense. Awesome temperature for an RPi4. Which fan du you use?

btw you could recall the banner using dietpi-banner 1

28’C sound like water cooling. On my RPi4, I never get below 38°C (running 24/7). Even with a constantly spinning fan.