Efficacy of RO systems is highly dependent on incoming water pressure and temperature. I got a professional water test done on our RO system and was surprised at how much remained in our (hard well) water. It turned out that our incoming water temp was far too low for the system to reach peak efficiency.
Another aspect is TDS creep. The membrane only reaches the specified rejection rate after a few minutes of use. If they under sink RO system is used frequently for small amounts of water it can cause frequent cycling of the RO system which will reduce the water quality produced.
My RO system can take the 450 TDS tap water down to about 30 under my normal use. If I close off the tank and run the water for about 10 minutes it will get down to about 20.
Yes, its a problem with the technology, so anything using RO will have the issue to some extent. Fancier systems will automatically flush the filter to help prevent this, but its rare on consumer grade equipment.
