[Biscute]

However harvested filtered pure rainwater does not leave spots. I wonder why that is?

Rain is quite pure nearly 0 tds. Most of the tds is picked of from the water being ran through miles of pipe line to come out your tap.

[Nathanael Jones]

TDS meters are very basic - they tell you that there are impurities in your water, but not what type of impurities.

Some dissolved solids spot/streak - but there are some things which increase TDS but do not affect cleaning. It really depends exactly what is in your water!

I reckon the number of ppl who can achieve that kind of result with water straight from their tap is very small though,..