There are many reasons for that and one funny one might be that they're dead. Now, I was kidding with that, but once in a while people might take vacations and leave their Steem account without doing anything, some might just keep their Steem tokens on Steemit as on a wallet even if they powered up while others are simply in between delegations or active and inactive times.
Over all I don't think that we have many such users, but you spotted good, there are some leaving their Steem Power unused and I just wish I had a delegation on that Steem Power.