BP Not showing Topic Count Correctly (Missing Some Out)

Bit bizarre, but I have noticed in the Forum diretory today an amonoly.

I currently have 815 groups set up on my site with 1 forum topic PER group.
This makes 815 topics too.

The Forum directory states that Total topics are 815, My topics are 815 but then shows underneath that it is displaying 1-20 of 814 topics?

Here's a grab. Any ideas what this is caused by. Seems a bit bizarre to me.