If the KH is 9, and you have no injected CO2, the ppm of CO2 in the tank will be around 3 ppm, or near that. So, the pH will be about 8. If you now add about 30 ppm of CO2 the pH will drop by 1.0, and will be about 7. In order to have a KH of 9, you have to be able to dissolve about 160 ppm of carbonate in the water, so the limit is the solubility of the carbonate compound involved. But, bicarbonates are very, very soluble in water, so that will give just about any KH you want. And, as the pH drops with addition of CO2, carbonate compounds become more soluble too.

I think what this says is that in the range of concentrations of CO3 or HCO3 and CO2 that we are interested in, there is nothing in to prevent the pH and KH from assuming whatever values are needed.