Ask your local fish shop how they make up their water. TDS can be made up of any disolved solids e.g. fertiliser/nitrates even stuff like dechlorinator or stress coat adds to it. So you can match TDS and still have a big difference in GH/KH/PH.
My local tap is about TDS 400, but when I buy fish their bag water is closer to TDS 800! I would guess that's some additives like stress coat, plus any ammonia that's built up in the bag etc. I mix my tank water from RO, so my tank runs about 200.
I wouldn't mix my tank up to 800, but I have been considering using tap in my quarantine so it's closer to what I imagine the LFS water is based on.