If your driftwood is leaching acids into the water, what will happen is exactly as you are describing, the water will test more acidic than it should be according to the KH. Driftwood will not remove hardness (KH, GH, or anything else). It's just adding acids which will mess with pH alone. The chart was not made to accommodate these types of variables, that's why a drop checker works best (using distilled water that has nothing in it to mess up the equation).
Also a drop of 1 pH when the water sits out will not necessarily mean 30ppm co2. It will mean that you have 10x the amount of co2 in the water before you let it sit out than you do after you let it sit out, but how do you know what you had originally? If you had 1ppm originally, then you have 10, but if you had 4ppm, then you have 40, so it's a pretty big margin of error there. It can tell you that you're doing something, but it can't really tell you how much.