The water you are using may not be truly "empty" water. For the most accurate results, test your water before and after adding TWW minerals. Starting water should read at 0-9 TDS.
Potassium vs. Sodium
The TDS meter is calibrated to read potassium rather than sodium. Potassium has a much higher multiplier vs sodium (~0.85 vs ~0.5 respectively), which is why you are reading higher TDS. If you are able, switch your TDS meter to read sodium.
TDS Meter Calibration
The TDS meter is no longer calibrated. Third Wave Water TDS meters can easily be calibrated. See the video on the TDS meter product page for step-by-step instructions.