Most seals are directional- in the case of wheel hubs, they are designed to keep grease in, since they are not normally submerged in water, espescially salt water. As pressure is applied to the inside of the seal, it will grab the seal surface more tightly. Pressure applied to the outside of it will be able to penetrate the seal. That is the rationale for turning them around- water can't get in and the grease can get out.