Wow really???? I believe you guys, but I know for a fact that the RO water I get from my LFS is around 5, he told me so and practically wouldn’t let me leave without buying some buffer. I got home and tested it, and sure enough it was around 5.2. Since then, I always test the pH before I put it in the tank.
Maybe the stability of your tanks is good enough that the lower pH of the top off doesn’t matter. I would have thought that even adding a few gallons of pH 5 water to a 50 or 100 gal tank would be enough to notice a change in pH.....