I posted in the other thread posted here, but this has been much more accurate for me. I don't get different readings like that. I follow the instructions to the T. First I make sure the vial is facing the same way the whole time. I shake the vial for 2 minutes exactly, inverting it slowly the last 10-15 seconds to get microbubbles out. It's important to wipe off the vial and to also rinse/store with water inside. If you don't, the glass gets stained. The checker won't be able to tell if the glass is stained with the c1, just that light passes through properly. It just knows that, if "x" color, then this.
I'm not saying you're not doing these things, but you won't get consistent results if you don't.
I'm not saying you're not doing these things, but you won't get consistent results if you don't.