It's probably a meter designed for high ionic strength solutions.
RO water is by definition a very LOW ionic strength, and it takes a more sensitive meter to read it perfectly. Your buffers, on the other hand, would be a very HIGH ionic strength solution, and your meter can 'see' the number more clearly, like a book with bold print.

If I were you, I would not concern myself very much with the initial pH of the water from your system. Check the ppm coming out instead, to make sure that the system is working, and then add your calmag and nutes, and THEN check the pH. By adding fertilizer, you raise the ionic strength to the level that your meter, which is a good one btw, was designed to read accurately.

Hope that helps.
stinkyattic Reviewed by stinkyattic on . New RO system raising ph Just picked up a 100 gpd ro system. Ran it for about 40 minutes first to clean it out like the instructions said, then filled up a 5 gallon bucket. Ph'd the water after and the filter raised it from the 7.3-7.5 range to 10. Is this normal? Do I just need to let it run awhile to clean it out? Rating: 5