Zero water is not pure RO water.
The ZeroWater filter contains ion-exchange resin and activated carbon. ZeroWater relies on the interaction between the ion-exchange resin and your tap water to make filtered water with zero conductivity. The TDS reading of 0 you see when you use a ZeroWater filter means the water has no electrical resistivity or conductivity. It doesn’t mean the water is contaminant-free.
Reverse osmosis systems are multi-stage filtration systems that use sediment, activated carbon block, and granular activated carbon prefilters before the reverse osmosis element or semi-permeable membrane. Your tap water is forced through the 0.0001-micron membrane. 0.0001 micron is equivalent to 0.00000004 inches. This tiny micron size is the reason reverse osmosis removes everything from your tap water, including essential minerals such as calcium and magnesium.
Zero tap filters leave some stuff behind.