KH Director Accuracy VS 7.5DKH Buffer AND adjustment feature

Just bought a new KHD probe and wanted to make sure I would have the 0.1DKH repeatability. I firstly calibrate the dosing pumps and ensure they were within 1% error on 4 consecutive tests (weight) and calibrate the probe. Then after 10 consecutive KHD test (100ml sample) with a brand new 7.5DKH buffer solution (at 25 C) I Have my 10 reading between 6.9-7.0 so was pretty happy to validate my repeatability was within .1DKH.

I would like to understand why though the readings are not as the buffer or within the range. The buffer is 7.5DKH +/- 0.2.

I said to myself no worries I can use the adjustment feature but it didn’t work as expected. I would have to make a plus 7% to make the 7dkh read 7.5 dkh (the value of the buffer) but it only made the next readings to 7.1 dkh !? And why in the troubleshooting and tips manual you say that this feature will modify the KHD lab grade accuracy?

How should I act doing nothing and just take into account that my KH Director reads -7% from the real value of the water it measures?

Many thanks and best regards