I don't think there would any shaft diameter difference, and it's not likely that the shaft is the problem. The shaft has to fit the bearing behind the seal.
It is possible for the bearing to be worn enough or have a broken cage to allow the shaft to become eccentric during rotation and allow for oil to leak past the seal.
Grab the shaft and shove it up and down, checking for slack, this will give you some indication of bearing problems.
Also- make sure the lip of the seal hasn't folded under upon installation.