If, after ensuring your Z frame is square and the X axis is parallel to the bed, you still have a difference between the left and the right side, it is probably because your printer has a small twist on the X axis. The X axis has two rods and any difference between the two will be amplified. Here is a model of the X rods and of the complete head with a BLTouch:
This diagram explains what happens:
Note: Thank you to @MauroGil for creating this model and diagrams.
It is a mechanical problem that is not easy to solve:
- Some people are pushing on one of the supports of the x rods
- Some people are releasing the external screws that support the X rods and retightened them in the correct position:
Since the version 5 of ADVi3++, there is a tool to help you measure this twist and compensate it:
- The
L,MandRbuttons represent the left, middle and right side of the x axis. - The principle of this tool is to measure the difference in height (z axis) between those three points along the x axis with the help of the nozzle of the printer.
- If you have performed correctly the Z-height part, at the middle
Mposition, the nozzle should be just above the bed (use a feeler gauge or a sheet of paper). - If it is not the case, you can adjust the height with the
and 
buttons. You can select the amount of the move along the z axis with the Increment buttons. This is exactly like for the Z height panel. - Then do the same for the left
Land rightRpositions. 
Once it is done, tap on the Checkmark in the top-right corner.
Tap on the Back button to cancel the action and go back to the previous panel.
Important:
- In general, this tool should be used AFTER the Z-Height step and the Automated leveling.
- As for the Z-Height, it is recommended to use a feeler gauge to determine the right distance of the nozzle above the bed.