If you take distilled or DI water and add sodium bicarbonate (baking soda), you are adding carbonates to the water. Now if you measure the KH and the pH, you will be able to use the table to determine how much CO2 is dissolved in the water, and it will probably be about .5 to 4 ppm. That is from atmospheric CO2 dissolving into the water. If you take a piece of airline tubing and use it to blow bubbles in that water for 15 minutes (don't pass out!), then measure the KH and pH again, you will probably find about 10 ppm or more CO2 in the water. Those will be accurate measurements, or at least as accurate as you can measure the pH.
Now if you take tap water, which may have phosphates added to raise the pH, put it into the aquarium where a piece of bogwood is seeping out tannins, and then try to measure the KH and pH to determine how muchCO2 is in the water, you will likely get a number around 50+ ppm, even if you aren't injecting CO2. That number wont be correct because the phosphates and tannic acid are affecting the alkalinity and acidity.
That's why a drop checker with distilled or DI water in it will give an accurate measurement of CO2, but not if tank water is in it.