Ah I see. I am one though that doesn't believe fish die solely from high CO2. From personal experience and reading of others fish can live just fine at high levels. It is other problems that coincide with high CO2 that kills the fish.
If CO2 is getting very high that a sign there is no suface agitation. If plants are not photosythesizing well then oxygen will become depleted - fish die.
Or, since getting high CO2 usually happens by mistake and lowers the pH greatly, the quick and drastic change kills the fish.
Anyway, your KH seems like it could be off compared to your GH which is very soft. Do you use a water softener? Try resting a small sample of water for 12-24 hours. If the KH and pH give you around 3ppm of CO2, then your test kits seem fine.