Test kits for KH and pH may be very accurate, but that is not the problem. The table that shows CO2 versus KH and pH is only accurate for water that has only carbonates affecting the KH and only CO2 affecting the acidity. Our aquarium water very often contains phosphates, which affect KH, and tannic acid, from bogwood or decaying plant matter, which affect the acidity. So, the CO2/KH/pH table doesn't give a good reading for how much CO2 is in the water. Unfortunately, the reading it does give is almost always too high, usually much too high.

The drop checker works because it uses distilled or deionized water, with a bit of sodium bicarbonate in it to raise the KH to 4 dKH. That water has nothing else in it to affect KH or pH except the CO2 that mixes into the drop checker water from the tank water, so that the tank and drop checker have the same ppm of CO2. Now, the pH indicator reagent in the drop checker will give an accurate indication of how much CO2 is in the tank water. The reason for using 4dKH as the KH of the drop checker water is because at that KH, with 30 ppm of CO2, the pH will be 6.6, and at 6.6 pH the color will be green, a relatively easy color to judge.