Why is my reverse-osmosis water not pH neutral?

With a clean unit, r/o should be as close to 7.0 and 0 ppm as you can get.
There are two reasons that the pH of your Reverse-Osmosis water would not be neutral:

1) Too high-- likely a calcium buildup in the filter, machine needs cleaning or replacement filters.

2) Too low-- dissolved carbon dioxide will lower the pH. It has almost no buffering capacity, meaning one tiny drop of pH up will put it back into place.