Anything is possible.
I would recommend getting Shrimp Minerals GH+ (not GH/KH+).
It will give you more flexibility. If you need KH, use your existing bottle.
Mix 50:50 of GH+ and GH/KH+ and you should get closer to the desired levels.
That way the current bottle isn't a complete waste.
That's because your test equipment is not accurate. API GH test 6 is not exactly 6, it's a range. Reading a GH value by counting the number of drops is never an accurate measurement.
GH as a measurement isn't any better than TDS. They are both valuable tools. Relying on one and ignoring the other is not a good idea.
GH test kits primarily test for Calcium & Magnesium. It does get influenced by other minerals like iron, potassium. But it's largely a tool to test Ca and Mg.
TDS test for a larger set of minerals, chemicals, and "stuff" that is dissolved in water. TDS can never equal GH. Or if you are converting GH to ppm = GH x 17.9.
Why? because TDS is testing for a whole lot of other things in the water. It will test the presence of Calcium, Magnesium, as well as phosphates, potassium, iron, carbonates, chlorides, sulphates. It even picks up ammonia, nitrite, nitrate. It also picks up metals like copper, mercury, iron that is dissolved in the water. Heck it evens pick up readings if you mix ketchup into the water. It reads anything that will dissolve in water.
So when you say your GH is a perfect 6, yet TDS is 300 ! Well it's time to do a water change, because you might have Nitrates building up. If you went by just a GH reading only, you would think everything is fine in the tank not knowing Nitrates is building up.
Likewise with just relying on pH. If pH is an ideal 7, but TDS is 400. Is that water fine for shrimp?
Most on SKFA will say no.
Your pH is fine. but the water is filled with other none desirable dissolved solids that will harm the shrimp or fish. The question is - what is in that 400 TDS of water that some suppliers send you? Some calcium, and magnesium in the water maybe - harmless. But more likely a huge amount of Ammonia, or Nitrates - harmful. The shortfall of TDS is that we don't know exactly what the TDS meter has detected. But you miss knowing the water could be harmful and full of dissolved solids if you didn't test TDS.
It could be beneficial Calcium. Or it could have been harmful Nitrates or Copper, or a list of other undesirable chemicals in the water.
That's why we use TDS as a guide, much like we use GH, KH, pH as a guide. These only measure ONE aspect of the water in your tank.
Use TDS as another guide to let you know that dissolved solids are building up. If you haven't added any calcium or magnesium recently, then it must be nitrates building up. If you add fertiliser for the plants, than measure how much before and after adding ferts. You'll know the next time the water needs more ferts added if it falls below your base TDS figure. You can use TDS to acclimate new fish and shrimp - take a reading of your tank, take a reading of the water the new fish/shrimp arrived in. Drip water into the bag until bag water TDS = your tank TDS - acclimation done! It's a lot quicker than measuring pH.
Hopefully you see my point, and I have convinced you that relying on one test is never enough.
Use all the tools at your disposal, or you face missing the big picture.
Saying GH is more important than TDS is just foolish. TDS offers you another aspect of your tank that GH does not. Likewise, GH offers you insight specifically to Calcium and Magnesium levels that TDS doesn't.
If you want to read more about TDS, I have a great article on TDS and why it's important somewhere on this forum.