Screen resolution for virtual displays

[UncleBoarder]

A failed monitor caused VirtualBox to change my "full screen mode" to 1 screen... even though I still had 2 screens connected and working without problem on the Ubuntu 14.04 host, VBox decided my max display size was ONE screen.

Here's the original [SOLVED] problem if you're curious: viewtopic.php?f=2&t=74660

My question is, how does VirtualBox determine the maximum screen resolution for the virtual screens? My host knows it has two screens 1920 x 1200. But my guest thinks it has one screen 3840 x 1200. Yet, surprisingly VirtualBox shows my virtual screen resolution as 1920 x 1200.

Yes, VBox says it's single virtual screen is 1920 x 1200, yet the guests see it as 3840 x 1200. All works perfectly. It's not just stretched over the 3840 width, it's actually displaying 3840 pixels wide.

VBox 5.0.2
Guest Additions installed
Ubuntu 14.04 host
Windows XP & Windows 7 guests