Second time an xml object gets displayed produces different output. Looks like the object has been changed just by trying to display it. Get-Member returns different results, output is different, don't know what else is changed... If anything but a leaf from the xml object is displayed on the console - looks like the object gets changed and the second time result is different.
Tried with PowerShell 3 (Windows Server 2008 R2 Standard x64, Windows 7 x64, Windows 8 Pro x64) and PowerShell 2 (Win XP x64) - same results... Haven't tried with PowerShell 1.