There are plenty of factors that could cause the amount of available video RAM to vary. On a modern OS the window manager may be using some of the video RAM. On windows Vista or higher video RAM is virtualized so results can vary. Video RAM may be fragmented from previous runs so free space can be affected. With anything that uses a dedicated/shared memory architecture (e.g. Intel) you'll get a figure that may be just the amount of dedicated RAM left (i.e. quite low) or may be the total of dedicated + available system memory, or anything in between. Basically this isn't a figure that you should be basing crucial decisions in your program on; instead it seems better to me to use rough ballparks (e.g. with GL 3.x level hardware things are fairly well standardized at the ~1gb mark or higher).