To be honest, i think that number system is a load of cow droppings...however, it's a number people are goign to be trying to increase all the time now...so...*sigh*...
You'll find the video card has both 2D/3D parts...different hardware.
the Desktop utilises both of these. 3D for the areo functionality, and the 2D for all the text, websites, etc etc.
The games have only the 3D area to use these days.
This is my thinking of it anyway

I could be completely and utterly wrong...