As @aggroed has mentioned, it's likely that some accounts (probably the larger in general) are owned by Steemit staff. Tough to find this information though.

Some accounts will be alts., and as I mentioned to @crokkon, voting on content gives clues here.

All things considered, perhaps most of the active / non-Steemit staff, honorable (errr :) ) accounts are voting for witnesses, and what we have left is a deluge of inactive, bot, test, lost, etc accounts.

More questions raised than answers, again :)