Andrew, if you look at, for example, the profile for Sir John Slade, First Baronet Slade, down at the bottom of the page, it says:
John is 20 degrees from Claude Monet, 22 degrees from Gigi Tanksley and 10 degrees from Victoria of the United Kingdom of Great Britain and Ireland on our single family tree. Check your connections or find your genealogical relationship with John.
Those three profiles*, in a sense, define being "connected": if a profile has a connection, however long and winding, to any of those profiles, then it is also connected to 15,169,742 other profiles. This is what we call "the main tree".
There is a list of the next largest unconnected branches on the Connectors Chat page. As you can see, the next largest unconnected branch is several orders of magnitude smaller than the main tree. I have often fantasised about mapping the assorted unconnected branches relative to the main tree, but I eventually realised that the information needed to show them in relation to one another (aside from sheer size) would be the same information needed to connect them to one another, so if we're going to do the one, we might as well do the other.
As to why Leslie's profile had [[Category:South_Africa,_Unconnected_Profiles]] on it (I don't see it there now), there are so many unconnected profiles (3,661,082, as I write this) that lumping them all together into [[Category:Unconnected Profiles]] would make that category so large as to be unwieldy to work in. Besides that, most connectors specialise in particular places (not just because they prefer to work on their "home ground", but also because they tend to have more expertise in that area, and better access to resources which relate to it). Therefore, where we can, we sort those profiles by country, and sometimes by county/department/province/state, so that people who work in that area can find and connect those profiles. (Personally, I work most frequently in British Columbia and Nova Scotia, Canada.)
In cases where somebody was born in one country and moved to another, we normally link them to both categories, in case somebody can make a connection in either place. So, in Leslie's case, he should have been categorised under both [[Category:Kent, Unconnected Profiles]] and [[Category:South_Africa,_Unconnected_Profiles]], and if the person doing the categorising was paying attention to the biography, they should also have added [[Category:Southern Rhodesia, Unconnected Profiles]], which didn't exist, so I've just created it.
* Actually, only Queen Victoria stays the same from week to week. Claude Monet is this week's profile of the week, and Gigi Tanksley is this week's member of the week. Those change from week to week, but since all 15,169,742 profiles in the main tree are connected to one another, it pretty much doesn't matter who the defining profiles are.