"vb_pending_queue_drain" stat doesn't seem correct

I think there may be a bug here in some of our stats. Looking at this issue live, I see the vb_pending_queue_drain stat constantly increasing (as in "I'm writing to disk") but I don't see the vb_[active|replica|pending]_ops_create increasing at the same rate...

So is this a bug? What process would cause the drain stat to increase but not result in "creates" to the database (no updates are going on either)?

I've attached a snapshot of the stats taken a few seconds apart...