In post #31 in this thread:
https://www.ford-trucks.com/forums/1...rebuild-3.html
the ball on the axle tube outer end is shown, and the opening that accepts the drive shaft. Possibly the drive shaft just is close enough fit to the axle end opening that thick knuckle lube doesn't migrate toward the axle tube.
When I took mine apart after at least 45 years, both axle tubes had quite a bit of semi fluid grease in them. This is why I assumed the knuckle lube didn't stay just in the ball containing the drive shaft U joint. On my axle it didn't come from the center section as it wasn't gear lube. It had to have been filled from the knuckle fill ports.