OK, this may explain things. First, do not use the high pH test, use the regular. As I said they do not provide identical results for the same water.
Second, there may well be something calcareous in the tank that is increasing the GH, as I believe another member mentioned earlier in this thread [I had assumed you were testing tap water on its own]. If the substrate is composed of calcareous sand or gravel or you have such rocks in the tank (calcareous includes marble, limestone, aragonite, dolomite, coral, shells) they will continually and slowly dissolve mineral (calcium and magnesium) into the water, raising the GH, probably KH (though this might be scarcely noticeable) and pH. This can also occur from some plant substrates, and some plant additives--I just mention this to have a complete picture. Could also be other additives that might do this, just thinking out loud.
When you test the tap water on its own for GH, does it agree or is it closer to the water authority's level?