Different VRAM sizes in OS X and Apple hardware Test

I have some problems with an external monitor by HP connected to my mid 2012 MacBook Pro (13"). There are white moving dots all over the screen when I use 1080p. 480p and 720p works fine.

I connected my MacBook to a TOSHIBA TV and it didn't work aswell. And I also changed the Mini Display Port to HDM adapter, but still not working.

Then I did the Apple Hardware Test and it didn't find any errors. But in the information there it shows "Graphics: Intel HD Graphics 4000 VRAM: 256 MB". As I know this should be 512 MB.

Can someone explain this all to me?