screenRect and screenLoc weirdness

[Knapp Martin]

So I just had a customer complain that windows were opening in odd locations with his multi-monitor setup: a MacBookPro with a Thunderbolt display. He has his displays arranged so that the bottoms were aligned with the 2nd display to the left of the MacBook.

So I hooked my laptop to my TV and discover some weirdness. I started with my displays aligned to the top and then got the screenRects and the screenLoc:

Main screen rect: 0,0,1280,800
2nd screen rect: 1280,0,3200,1080
screenLoc: 640,400
(all as expected)

Then I realigned the screens so that the bottoms were aligned:

Main screen rect: 0,280,1280,1080
2nd screen rect: 1280,0,3200,1080
screenLoc: 640,680

Why would the arrangement of the screens effect the screenRect and screenLoc of the main monitor?

Marty