Why is the wheel bearing grease leaking out of the dust cap?

I do alot of 4-wheeling, dirt biking, general off-road stuff Jeep stuff.
Ever since I switched to Bel-Ray waterproof grease I have never lost a wheel bearing or a u-joint.
It seems to be good stuff, a little more pricy than regular grease, but bearings and u-joints are not cheap.