TDS can be made up of many different elements as i'm sure you know. i work in a lab that tests municipal water and Yes, it is very common for water to change in the amount of TDS depending on the time of year and what is happening with the system. I assume your units measure TDS by conductivity, conductivity being the ability of water to carry a current. This method simply uses a calculation (0.67x#microsiemens) to determine TDS.
I don't know much about RO, but if your unit is not producing water with a no TDS wouldn't you want to either slow the water down so that it can be deionized better or replace the membrane and/or solution?
Our tap water here has a conductivity of <20, so very low TDS. Reagent grade water in our lab has a conductivity of <1.0, so a TDS of <1.0 is very very negligable. I mean, we have equipment that tests for calcium, magnesium, iron, and all the other anions and cations and we use water of 0.7 TDS without affecting our test results.