Consider the following scenario:

You have onboard UMA 384 MB + 1GB from the video adapter and you expect the total video memory to be about = 384MB + 1024 MB => 1408 MB

However, in the Control Panel -> Appearance and Personalization ->Display ->Screen Resolution Panel, click [Advanced Settings]. Under the "Adapter" tab, you will see the Dedicated Video memory value displayed as 1536MB.

UMA DSC_384

Why?

Basically, the UI is coded to display the Dedicated Video Memory in the form of power of 2. So, it uses this function “NextPowerof2(uNumMegs/8)” to calculate the allowed adjustment. And here are the allowed adjustments for different ranges of memory:

//  129M-256M rounds in multiples of 32M

//  257M-512M rounds in multiples of 64M

//  513M-1G rounds in multiples of 128M

//  1G+-2G rounds in multiples of 256M

//  2G+-4G rounds in multiples of 512M

And we can see that for the case of UMA_384+DSC(1024MB), the adjustment is in multiples of 256MB. It is not possible to display 1408MB. And since 1408MB is greater than 1280MB(256*5), that is why you are seeing 1536MB(256*6) by design.

This power of 2 algorithm was implemented per KB938838.

Windows Vista displays the incorrect dedicated video memory size for certain display adapters

http://support.microsoft.com/default.aspx?scid=kb;EN-US;938838

To display a more accurate video memory figures, use the videomemory.exe tool from the DirectX SDK tools. Link:
http://msdn.microsoft.com/en-us/directx/aa937788