When I use my Fluke 189 to measure 10.000V DC,
The measure reading is 8.888V , I feel lucky with this number but I still want to fix it

Since it didn't happened all the time, It's really hard to find out the root cause.
I checked all the input protection and it seems they are fine.
I have clean the gold plated trace under the rotary knob as well but the problem remain the same.
Actual input 5V
Reading 4.8V
Actual 10V
Reading 8.888
Actual 14V
Reading 13V
Actual 19V
Reading 18.010V
Actual 20V
Reading 19.038
Actual 21V
Reading 19.076
Actual 22V
Reading 20.643V
Actual 28V
Reading 26.6V
When I use the 189 to measure 20V, it will display 19.038V and
When I use the 189 to measure 21V, it will display 19.076V only
I wonder to know which component will likely cause this 0.04V different on the Fluke 189 meter in that specific voltage range?
Please advice,
Thanks!