GH is the measure of calcium and magnesium (Ca++ and Mg++) ions in the water. It also has nothing to do with iron. Well water does tend to be high in GH.
There could be a lot of reasons why the KH reading is so high. Being exposed to the ground/rock is the most obvious. I do believe the test results should be attempted with at least 2 different testing kits. (Doing only one other test can only indicate a difference, not indicate which result is accurate). I would also suggest testing a sample, both tap and aquarium, that has been drawn 24 hours earlier.
I also don't believe that your KH can be too high. It makes little difference to most fish or plants. Some fish may not attempt breeding in less than optimal conditions but maintainance should not be a problem. A high KH means your pH should be rock steady, resisting any attempt to shift it.
HTH
PP