In public land, there are no plots calculated/loaded. So if you were to type /realm it wouldn't show you which realm you're in. Just open land with no pre-determined grid.
Then, when you do /realm purchase, the server temporarily refers to the grid which we can't see and calculates which realm you would be in?
For example, player A is at some point far from spawn. Upon doing /realm purchase, the server determines that he's in the realm at x3275 z4075 and he now owns it just like before 4.3?