Easy. It's all to do with the velocity of a parcel of air in the direction of a line of latitude.
If the parcel of air has come from the equator, said velocity will be high. If it's come from the poles, said velocity will be low.
The apparant track of each parcel of air over the surface of the earth depends on the velocity of the parcel of air compared with the velocity of the surface of the earth where the air has ended up.