Edit: You found a bug, it's because you had a selection between the coordinate corners. I'll fix this!
This is acutally a very crazy bug. If you are in a high coordinate area near to the border and select a point on the other side of the border, the coordinates count further. For example you are standing at 998,50,100 and are selecting a point behind the x-border, the selected point becomes something like 1002,50,100. But if you select the same point from like 10,50,100 it becomes 2,50,100. It is working the other way, too: Selecting from 2,50,100 at the other side of the border it becomes for example -2,50,100. If you are selecting an area with point 1 on one side and point 2 on the other side of the border but doing this from one side of the border (like standing on the same block for both selects), everything is working fine. The area is something like -2,50,100 to 5,50,105. But if you select one point from one side of the border and the other point on the other side, your area is like 998,50,100 to 5,50,105. Obviously in this moment you fuck up your whole world (if you don't realise this error soon enough)