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.