Pressure on two sides of a surface don't have to be equal. The overall forces do, but there's an additional component being applied by the surface itself. An alternate way of looking at it : consider how the space station internal pressure differs from space.
> Pressure on two sides of a surface don't have to be equal.
Absolutely, and Newton surely didn't say so. :P
(A more close to home example is that the pressure inside a tire surely doesn't have to be the same as the pressure outside the tire. In fact, if it is then we call that a "flat tire". :P)
But the force that the tire exerts on the road (=the "action" as Newton called it) has to be equal to the force that the road exerts on the tire (=the "reaction" as Newton called it). Since the contact surface between tire and road is the same that means that the "overpressure" in the tire (=the pressure the tire exerts on the road) has to be the same as the pressure that the road exerts on the tire.
