Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The TDS results look good. It is common for the tanks to have more stuff in the water than the tap (in this case, the well) water.
Yes, double check the GH and KH. Maybe both are off!
Most of the calcium, magnesium and carbonates in water (especially water that has been in contact with the ground, like well water) come from minerals like Calcium carbonate, and magnesium carbonate.
When these minerals break down, or are dissolved in water, then both the GH (Ca and Mg) and the KH (carbonates) rise.
There certainly might be some other source of any of these, some minerals that just contain carbonates, but not Ca or Mg. Or there might be something removing some (the Ca and Mg) and leaving the KH.