Some bearings may appear to be sealed it doesn't mean they are air tight. Sealed, in the truest sense. That will keep grease in and contaminants out, and the over zealous bike washer. Some bearings appear to be sealed, until you inspect them closer. I can see how some of the later bearing retainers can get mistaken as shields. I'm not sure where every misconception gets started but if I was a betting man I know where I'd place a few. Then some bearings are called sealed, and are sealed by design. It can be a bit of a misnomer. It isnt meant to misguide anyone or limited to bike bearings but it can be one thing that aids misconceptions.