The lower the grease fill % the easier the bearing will spin while not under any load. The common thinking then is to use a very low %. Some companies do this for one reason- to make you think their bearings are faster than they are. They know the first thing you will do when you install your wheel bearings or BB is to spin the wheel or the crank and see how long it spins. This is a great way to see how good the bearings are, right? Not really.
A brand new bearing will spin mostly based on the amount of grease, the tightness of the seals and how heavy the spinning object is. A heavy wheel/rim, tube and tire will spin longer than a lightweight wheel/rim, tube and tire.
Knowing this, most bearing companies apply a low fill % of grease and use light seals, since you will think the bearing is awesome because it spins so long. But in reality, this is a bad practice. You need enough grease to properly lube the bearing and help it's longevity. You need proper seals to keep the bad stuff out (grit and water) and the good stuff in (the grease).
Plus, spinning a wheel or BB while unloaded (not being ridden) completely misses the point with how bearings work. They work when loaded and the main reason our ceramic bearings are so well received is because of the quality ceramic balls that we use. Our round balls spin easier than less round ceramic (lower grades) and even more less round steel balls. A rounder ball will not produce as much heat like a lower quality ball, which means the rounder ball spins easier. Matched with our races, the bearings spin like on a glass like surface.
We tend to use slightly heavier seals and a slightly higher grease fill %. While it will take a little longer to break our bearings in, they do last longer and in the end, that is also why you upgrade to ceramic. While we use a higher grease fill % than most other ceramic bearings, our grease is specially designed for our bearings. It spreads easily while breaking in and isn't too tacky nor too watery. It is ideal for ceramic bearings and can be applied more liberally without causing a complete slowdown in the turning of the bearing.
What happens when our bearings are going through the "break-in" process?
3 things: First- as the bearings break in the grease gets spread around and pushed to the cage which reserves long-term continuous lubrication. A thin oil film forms on the balls for lubrication. The thin oil films will generate less drag on the balls. Second, the seals will break in and loosen. Third- The ceramic balls continually polish the races making them slicker and faster with time.