It is a HM TDS-4. It is brand new - this is the first time that I've used it.

I suspected the meter might be off, but it gives consistent reading +/- 4 on the tap water and on the pool water.

Say if it was off by a factor of 10 and the pool was 1570 and the tap water was 2600, that still doesn't account for the pool being less than the fill water.