After filtering, there’s still some limescale in, for example, a kettle – how is this possible?

It is worth noting that a BOROUX water filter does not completely remove limescale from the water. Larger amounts are reduced and synthetic limescale is removed. However, a BOROUX water filter retains beneficial minerals (those that occur naturally in water and are good for human health) in the water. And limescale (calcium) is one of these. So you retain the mineral calcium in the water, and this can still cause some limescale build-up. In short, a BOROUX water filter is not designed to remove limescale from the water.