I think I solved the mystery.

The gravel + water on its own yielded no rise in kh, and neither did
the CO2 into a bottle. But, when I was running the CO2 into the tank I
used a check valve. (I didn't use the valve when I ran the CO2 into
the bottle.)

Well, the check valve had some type of filtering material. It was the
only valve in the fish store and I didn't think much about it when I
bought it. I found another valve w/ no filter material and tried
injecting the CO2 into the tank again, and, no extra kh.

I feel silly because I hadn't noticed this before posting!