Thanks for reviewing this charger. For us Samsung users listing the PPS range would be very useful. For some reason manufacturers don't list it and reviewers often also don't - why is that?
And I wonder why you question its accuracy. That 3% difference is likely caused by the resistance of your USB Cable and connectors. Using the values from the picture I calculated around 250mOhms, which is in the normal range of a medium quality cable.
I have seen much worse accuracy with all sorts of displays in chargers and cables. I would consider this astonishingly good - my Anker chargers display is certainly way more off! In reality nobody really cares, because for checking if a good charging speed is selected a ballpark figure is more than enough.