Seems to me that the PH of water is determined by what is in the water. My understanding is that plain old pure water will have a PH of 7, while molecules in the water tend to raise and lower PH.
That being the case, I would figure that water out of marijuana's PH zone likely contains molecules that may, if present in substantial concentrations, interfere with the chemical processes that drive nutrient absorption and growth.
During the final phases of flowering, I believe the plant is still absorbing, transferring, and processing nutrients from the soil and itself. To say, though we may not be adding nutrients to the plant, the plant is not living in an environment devoid of nutrients.