I honestly don't know how they "seal" at all.
Water can get in through the center of the "cups", and It's not obvious to me that the O-Rings seamlessly mate to the machined surfaces on the bottom of the uprights.
BUT... My problem was not due to water intrusion, it was due to the inability to get grease into the bushing / sleeve interface for 4 years - even though I thought I was...
Rocky