RO water can actually have fairly low pH that can easily be lower than distilled water. The reason is that some water supplies have dissolved gases, such as CO2, in them. Dissolved gases can easily permeate through the RO membrane into the treated water. That can drive the pH of RO water very low. In most cases, the gas content is minor and there is little problem. Some water supplies have to be run through an air-stripping tower to reduce that gas content to avoid this problem.