GH test kits are not generally very reliable, based on my experience with them. API kits tend to give high readings, or never change color no matter how many drops you add, again in my experience. Other brands may be a little better, but at best they work well for a few months, then stop working.
It would pretty cheap to look for another GH test kit, not API, and see if you get the same GH. Or, best of all, calibrate a GH test kit before using it. I haven't figured out a good way to calibrate that kit though.
Another thing to do is check the water quality report for you tap water, to see what it says the ppm of calcium and magnesium are.