The sink assembly isn't intended to be a snug fit. Its intentionally built the way it is so that overflow water in the overflow chamber can easily reach the overflow holes in the assembly no matter its axial orientation.
Sealing of the sink is managed in 2 ways. The wide flange on top is just to keep water in the bowl and will only do so if its sealed properly. The wide rubber gasket on the bottom side is what keeps the sink from leaking. It seals both against the sink and against the drain assembly.