Because of the nature of a connecting rod, pistons move at different rates about Top Dead Center (TDC) and Bottom Dead Center (BDC).
Imagine a 2-cylinder "boxer" configuration. Cylinders are 180-degrees opposed and both connecting rods are on the same crank throw. At first it would appear that the volume of gasses in the crank case would simply move back and forth with the pistons. However, since the motion of the pistons are different, the gasses trapped in the crank case are actually being compressed.
Let's complicate matters further by adding six more cylinders. And yet again further by placing the cylinders in a V configuration. Now through into the mix, a dual-plane crankshaft where the rods are spaced at 90-degree intervals. The volume of the crank case continuously changes with each rotation.
If that wasn't complicated enough, even good sealing piston rings will have a percentage of leakage. Every time a cylinder fires, combustion gasses leak past the rings into the crank case.
Without sufficient ventilation, or better yet, evacuation the crank case becomes pressurized, increasing the stresses on gaskets and seals.
Now consider the rear main seal, it is being asked to maintain seal against a rotating shaft and to do so with a split design (not a continuous uninterrupted sealing edge). At 3000 RPM there are 2,159.7 surface feet of bearings journal moving past any point on that seal every minute. At 5000 RPM, the Surface Feet per Minute (SFM) is 3,599.5. It's a lot to ask of a little $3 piece of rubber, or worse yet, a piece of rope.
Some engine builders insist on installing the seal where the joint has an international mismatch with the mating surfaces of the block and main cap. The theory is that the path that the oil has to take to leak is more difficult than if everything is lined up.
Other engine builders prefer to align the joint of the seal with the main cap surface (this is the way the factory does it). The theory is that the ends of seal are pressed together harder, when the main cap is installed, forming a tighter joint at the ends of the seal.
Either way you choose, It is good practice to put a dab of RTV (I forget which color is most resistant to oil) on both ends of the seal to prevent any voids.
If you are still bearing with me after all of this, here is my current thinking on your situation: I'm thinking that you possibly have a bit too much oil in your pan, either allowing the oil to slosh up the back surface of the pan and cover the seal. Or you have a bit of a windage issue, again overloading the seal with oil. Excessive pressure from lack of crank case ventilation is blowing the oil right past the seal.
It is entirely possible that the seal has failed and/or was installed wrong, but I do believe that breather capacity is an issue here.
