Oh fer chrissakes - it is the RUBBER SEALS that "break-in' and feel smoother after a few hundred miles.

The ball bearings and races, even in a sealed bearing those also burnish in a tiny microscopic amount, but you wouldn't notice that.

Some seals are tighter than others depending on the level of "shielding" required for a given expected lifespan. I'm guessing that most Bicycle related sealed bearings are pretty basic for their seals.