Question for Steinway Techs

I have a new Hamburg Steinway 'B' which gives me great joy. The other day my
brother, who is a painter, noticed with his artist's eye that the keys go in
a gentle arc with the centre of the keyboard very slightly higher than the
ends; I hadn't noticed this previously but the arc is so perfect that it's
obviously intentional.

Any of you experts know why this is?

Stewart Taylor