RO -> removes impurities by filtration and in the process, also reduces TDS.
UV -> kills germs but doesn't filter, and therefore keeps the TDS levels intact.
Reviva -> RO and UV work separately and TDS controller combines them to achieve a specific TDS.
If the treated water has low TDS, it can be brought up by TDS controller. It just adds "UV treated" water to "RO purified".
You've already got such low TDS levels, which will be further reduced after RO filtration. If you try to bring up the TDS, you'll end up using UV treated water most of the time.
Water with low TDS tastes bitter, so they came up with this solution to make it taste better.