Thank you Patrick. You are right, my experience with “normal water” fits into your data.
Do you have any references for that data? I would like to read a bit more but can´t find it in the internet.
I just tested my rainwater and made a little experiment. I put some oak leaves and branches in and the next day the pH is about 4 at 37 µS. It seems that my water has less buffering ions than ever.
So your explanation is fitting.