I'm guessing because ceramics are hard, but not as hard as steel, so they're less precise as grinders.

Thing is, no grinder can produce perfectly-sized grounds 100% of the time. Due to randomness in how the beans break, there will always be pieces smaller than the desired size. (Probably larger pieces sneak through, too.)

Grinders all produce a distribution, it's just that burr grinders tend to have a tighter distribution than cheap spice grinders. I'd guess that ceramic burr grinders have ever-so-slightly more give than steel, and will let through slightly more undesired grounds as a result.

But it's probably a pretty minor effect.