KH testing, and GH testing, at least with the API kits mean count every drop of reagent, including the one that makes the color change.
CO2 can't possibly show up all over the tank instantly just because we inject it somewhere in the tank. It has to circulate with the water all over the tank, reach equilibrium with the losses to the air above the tank, and build up exponentially to the overall equilibrium in the tank. If you use a pH controller, and have the needle valve open well beyond where it has to be to get to the concentration you want, the time will be the shortest. And, if you inject the CO2 in an external reactor in the filter return line, with a big flow rate filter, the time will be the shortest. For me it takes a couple of hours at least to reach the final amount in the water, but that is as measured at one location in the tank. Other locations will reach the final amount sooner or later than that.