I would guess that they have a treatment system that isn't big enough, and they pump in unfiltered water to make up the difference when the treatment plant cannot keep up.

Even in the US I know of cities that have had boil orders because the treatment plant couldn't keep up and they pumped in untreated water. The cases I know of the unfiltered water made it in via weather events (the treatment plant is on the river and it flooded. Also a case where the power went out on a big surge and so they hardwired the pumps to the well bypassing treatment until the treatment controller boards could be replaced.)