IIS6 logs: stats=200, sc-bytes=0 -- any ideas?

I've been looking through IIS logs all day and discovered many entries which
look otherwise normal except for the "sc-bytes" field being zero. What
conditions would cause this to happen? It's not just one client or one
URL - it seems pretty random, and refers to normal documents that usually
have non-zero "sc-bytes" fields.

Thanks for any pointers!