Statistics Arrows

I'm having difficulty understanding the statistics which have no down arrows or up arrows. I understand that a green up arrow would mean that we've gained a node (perhaps a user), and the down arrow indicates that we've removed a node, but sometimes the arrows will appear by themselves with no number and sometimes there is a number without an arrow. Furthermore, our site users have no permission to delete themselves, and I have a firm grip on any admin activity. Is it possible the down arrows (even with numbers) are reporting some other statistics, such as user activity?