I'm not sure what compiler/IDE the Planetside people are using, but with Microsoft Visual Studio you can control the TabIndex property for each control (button, textbox, etc.) The operating system does have a tab hierarchy, but it is programmed too.
I believe the TabIndex sequence is set by the object creation order, but as I mentioned, it can be modified to a more efficient order.
The point is, the text entry box is a mouse click/keyboard event and the sliders are usually mouse driven. You make a decision on the type of data entry for one field and when satisfied you go to the next (I like using the tab key to do that.)
So if the TabIndex for Minimum scale is 58, setting the index to 59 for Maximum scale would accomplish this.
Of course there's occasions where you use a slider and then decide to fine tune with the text entry so a shift tab to back up would work. But I suspect that happens less often than a quick click/enter/tab/enter sequence for the example I mentioned for max/min entry.
So it's not a bug, but an annoyance, at least for me.