Mineral deposits from sprinklers, pool splash and the valley's hard water bake onto paint and glass. Light spots wash off. Baked spots do not.
This page explains what a water spot actually is, why the Colorado River supply leaves them, and how a detailer decides between a wash, a decontamination and machine polishing.
Water evaporates and leaves its minerals behind: calcium and magnesium on the panel. A fresh spot sits on top of the clear coat and lifts with a wash. A spot that has sat in the sun can etch a shallow crater into the clear coat itself.
After a wash, look at the panel at an angle in daylight. A shadow that remains is etching. A mark that disappears when wet but returns when dry is surface mineral, which a decontamination wash removes.
A fresh mineral spot often lifts with a wash and a dedicated water-spot remover. Etched spots need machine polishing, which is a paint correction service.
Etched spots remove a little clear coat where they sit. Repeated etching in the same place thins the layer, so removal happens sooner rather than later.