Different Plastics for Storing Water at 0 TDS

I have an ATO system installed, and it's been working very well for the past few days. I also tested my water for TDSs in the reservoir and they were up to about 52. Not good. As a reservoir, I'm using an empty 5 gallon salt bucket, well cleaned out with fresh water and dried before use.

For example, I put a gallon of fresh water at 0 TDS into a distilled water gallon jug and in a few days the TDS rose to 37. Nothing had been in the jug but distilled water...

Here's my multi-part question: Do different plastics leach TDSs into the water? The containers I bought from my LFS are plastic - - why don't they leach contaminants? I've used gallon jugs to store water (easier for me to carry) that had nothing in them prior but distilled water and they leached contaminants after a few days. What's up with the different plastics?

Thanks,
Leigh