After filtering, there is still some scale in e.g. kettle, how is this possible?

It is good to know that a Berkey water filter does not completely remove limescale from the water. Larger amounts are reduced and synthetic lime is removed. However, a Berkey water filter retains good minerals (which belong naturally in water and are good for humans) in the water. And lime (calcium) is one of them. So you keep the mineral lime in the water, and this can still cause some scale. In short, a Berkey water filter is not there to remove lime from the water.