In fact look up some tests of bottled water...lots of brands just fill from the same water system the public uses. It is not some miraculous healing water touched by god or something.
Bingo. Out of 200 something brands of bottled water being sold as mineral water on polish market only a dozen actually could be called that name. The rest, as you mentioned, fill from the same water system the public uses. It leads to a situation where very often people buy bottled water that is of lower quality than their tap water.