Entirely normal.
Generally Tank water TDS is higher because of all the biological processes in the tank that add to it. The tap TDS is low but not zero, you won't ever get zero out the tap because the pH will eat copper pipe, buffers are added to bring the pH above 7, this adds TDS, chlorine may be added and chloramine also to make the water safe to drink, this will also add TDS, mineral content (measured as GH and KH) are likely low but will constitute some part of the TDS.
My tap TDS is 30-40, municipal supply fed by reservoir caught rainwater coming of the hills.