Site Navigation Audience Targeting

Hi everyone

I'm playing about with the navigation audience targeting and have a query about the groups that are available to be added as an audience.  I want to add some links that are only visible for site owners but I don't seem to be able to choose any owners groups.  In the example below if I start typing the name of the site I only get the members group as an option, and if I type owners no results are found:

I wondered whether site owners always saw every link regardless of audience targeting and that's why they can't be selected.  However, I've tested this by setting a link to a group that I'm not a member of and even though I'm a site owner I couldn't see the link so don't think this is the case.

Does anyone know why I can't seem to select the owners group?  I'd be grateful for any help received.

Thanks in advance.