The reason is the guest caching, as covered earlier in this thread. As far as I'm aware, there's nothing youv can do, at least in terms of the way you're looking at it.
Re-framing it slightly though, maybe you could consider what you can do to encourage more of those guests to register? I'm not a fan of register walls and the like, so I'm not suggesting that sort of thing, but can you do something to make membership more compelling? Can you improve the messaging shown to guests so they're better aware of the benefits of joining?
All told, a member is far more valuable to you and your advertiser's than a guest, they're far more likely to revisit, can receive emails, notifications etc.
And for creating buzz, why not talk about the number of posts, new members joining, active members, trending topics and so on. I'd have thought those things are more likely to drive engagement than a simple 'look how many people are online' type thing anyway.