How does the router device have any clients with 'N/A' in their MAC address?

It seems that even with the 1.3.3 update today there are still times when the router is creating 'clients' (viewed by going to the ControlD website, clicking on Analytics, selecting the Router device, and clicking on the list of clients inside it) where they don't have a MAC address.

This means that even when there's a MAC-based policy for a particular device, if ctrld didn't identify the device's MAC address it treats it as having the router's policy. This is definitely not ideal. Is there something I might be doing wrong or some way I could tweak my system to avoid that happening please?