Understanding bed levelling  

Understanding bed levelling

Hi,
I'm trying to understand the bed levelling - I've done lots of searching, and I'm still not quite getting it I think.

My bed clearly has a slope from front to back. Mostly this has not bothered me and I've been printing fine for over a year with PLA.
I'm now using ABS, and printing larger objects.
I thought that the PINDA probe measured the height at the 9 points, then computed offsets in order to "level" the bed in the firmware.
(If it doesn't do this, what does it do?)
However, I've re-done Z calibration a good few times, and I always end up with more first layer "squish" along the front than the back.

I had a big print fail yesterday, so the time has come to tackle this!!

Reading through the instructions, all I can find is on live Z:
https://help.prusa3d.com/article/ZhBlGFD9Ah-live-adjust-z

I've updated the firmware, and run the first layer cal. This gets it right in one place, but then surely the PINDA ought to offset this elsewhere on the bed so it's right (or nearly) everywhere?

What am I missing?

Thanks,

James