How did you get the 7.0 value? I agree with mattrox -- the best way to check your pH is to let a cup of tap water sit overnight and then test. For example, my pH out of the tap is 8.2 ( 8O ) and after sitting overnight, it's 7.8. If I check the pH in the aquariums, it's 7.2. Established aquariums drop their pH value a little. I'm not sure of all the chemistry involved, but the acidic wastes of the fish help to lower the pH a bit in an established aquarium. So that's one reason your aquarium's pH is lower than your tapwater's pH.
The reason the pH doesn't get too low is that you do partial water changes and replenish the buffers that stabilize the pH a bit. But you also have driftwood. That lowers the pH too. I had a piece of driftwood in a bucket and I put it in tapwater and then tested about 4 days later. The pH dropped all the way down to 6.6. So driftwood is a pretty good at lowering pH.
To combat the effect of the driftwood, you could do more frequent water changes to replenish your water buffers. But this may be too time-consuming. You could consider fake driftwood or put some crushed coral in your filter, like Fishyfanatic said. You'd have to experiment with some
CC in a bucket to get an idea of how much you need to raise the pH. Remember to do it very slowly so you don't stress the fish.
What kind of fish do you have? (I can't see your sig from here!) Some tetras are fine with a pH of 6. You could check Liveaquaria.com to see the pH requirements of each fish. If you do decide to raise the pH with crushed coral, just do it slowly. If you decide to take the driftwood out, your weekly water changes should raise the pH slowly.