Powershell: Get-Process Returns "Invalid" VM Size

I'm running PowerShell 2.0 on Windows XP SP3 and I execute:

PS> ps firefox

And it returns:

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -开发者_运维问答----      ----- -----   ------     -- -----------
    859      44   340972     351580   684 9,088.22   7744 firefox

However, Windows Task Manager shows the following stats for firefox.exe:

Mem Usage: 354,720 K
VM Size:   347,322 K

Why is the VM output from PowerShell 300 MB more than that output from Windows Task Manager?

EDIT: It looks like the PM (PagedMemorySize) property output from PowerShell is lining up closely to the "VM Size" output from Windows Task Manager. However, MSDN says the PagedMemorySize property is obsolete. Should I rely on the PM property to get the virtual memory? What does the VM property represent?

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This post might help explain what the various memory columns in task manager correspond to. I've noticed some discrepancies when compared with Process Explorer too.