If GPU-Z shows the correct amount of VRAM then there is no problem. The problem is either that some is being hardware reserved, necessary for basic display function and not available for use by the game engine, or the game is simply reporting incorrectly.
You have a gremlin stealing tiny bits of video RAM to feed it's kids. You will probably notice it with your hard drive also, drive is 500 gb but it will show up at 470 or something.
Cats are good defenders from gremlins, I suggest 2 so they can work in shifts.
Or just ignore this since it's not an issue. You don't often see the exact amount of memory that is on the box in reporting tools.