This is just my own theory.
The lip overhang would enable the piano builder to place a spacer (Gage) under the lip, press on the key and easily adjust the height of each key, so that they are all uniform.
Regards,
Jerry
Could be...
But its not used in playing, i would love to see not to heavy piano weighted action with watterfall form factor... it could be the one keybed to fit all..