Well not really off the charts then I guess...
Several things can lower the pH besides carbonic acid, including temperature....
KH test kits actually test alkalinity not carbonate hardness. Carbonate hardness is usually the major part of alkalinity , but how big a role it plays is hard to say without more difficult testing. Another way to go about it is to use pure water and bee sure to add only -CO3 as a alkalinity source (the way the drop checker works)
Add peat or pH up-down solutions to your system and the chart will almost certainly not work for you.
Does the chart work ? Sure, it works quite well in controlled conditions. Our aquariums are influenced by other uncontrolled for factors. Nevertheless, it still offers a good aprox in most cases. Just be aware that when your result is 30ppm, the actual value is somewhere around (15ppm-45ppm).
I do not see that many problems with drop checkers. Once the right 4dKH solution and a good pH indicator is used all is left is to make sure the device is clean and not contaminated. The dropchecker will turn the color the test will turn once a specific pH is reached in the dropchecker solution. Does it have limitations, of course, like all measuring devices.
Now coming to your aquarium, if the drop checker is yellow and your pH is so low than I would say you have high CO2. It would be helpful to see what pH do you have when you degas the water. You do not have to wait a couple of days, just take some water out and put a working airstone in. In less than an hour the CO2 levels from that should have come at equilibrium with the atmosphere. This will tell you is your pH is influenced by other things.
On hiatus till later this year