The topology of molars is a fascinating thing. The ridges and valleys on the crown are already formed when it ruptures your gum; so in order for your bite to be comfortable, not only must the placement, along the gum, of corresponding upper and lower teeth agree, but also the topology of upper and lower crown pairs have to match snugly. It’s more complicated than starting a walking bridge at opposite banks of a river and hoping that the two sides meet in the middle.