You are not the only one. I had no idea what it meant also. Maybe a tooltip can help out on this... a tab 'should' be clear by itself, but in this case it isn't if we need to figure out/think about what it might mean.
As with anything, you can edit the phrases if you can think of a better one, but it's shortened to "Site" as "Categories" would be too long. It may change in the future if the layout of those tabs were changed.
I actually agree with you. But with slight improvements (here are some I posted myself) to the user-interface it could be made less confusing. For me the confusion mainly lies with how the software guides the enduser in interacting with the media. That could be made a bit more clear and intuitive in certain areas. That is why I love this one you posted yourself in relation to the [Site]-tab issue: big improvement!
Somehow in some parts the UI of XFMG is less clear and less intuitive compared to the truly outstanding UI/UX of Xenforo (forum) itself. I guess because it's developed by different developers. And so some things (some of the buttons for example) do not match with the main product. But I guess it's just the 'kinderziektes' (teething problems in Dutch) that needs to be smoothed out a bit.
At least we finally have User Albums in XF! I will buy this invaluable addition to the XF forum in any case. Let's face it, we waited more then 4 years (understandably, since XF needed to start from scratch etc) for XF to offer User Albums. And now it even comes with a Gallery, which will be a fantastic asset to the platforms that need it, like my board. I am sure many board owners are very pleased to see this development at Xenforo Ltd: a surprising but eagerly awaited new XFMG product and even a new developer.
(Now... the only thing I am waiting for is User Blogs, so we can finally switch from vBulletin to Xenforo...)