That's confusing as heck. I never heard of a bearing being sealed on only one side. It sounds like a manufacturer mistake to leave the seal off one side of the bearing.
"Hey, Joe! Don't you think we should install the seals on BOTH sides of the bearings?"
"Nah, the boss said to cut costs. We're doing our part."
Had a Ford pickup F-100, 1981 that didn’t have a rear axle oil seal on the passenger side......................