Well then, your GH test kit is inaccurate. You don't say which GH test kit is used.
Most GH test kits are only an approximation.
Take the API GH test kit for example. You drop the test solution into the vial until it changes colour.
The drop size is inconsistent firstly. And secondly, what if it took 6.5 drops to change colour? It doesn't measure anything less than the whole number. So the measurement is a range ... if the colour changed at 7 drops, then the reading is between 6 & 7.
What is your TDS reading?