MSI GT70 -Who Do I Believe?

I have an MSI GT70, with an Nvidia GTX 870M graphics card.

GPU-Z claims my GTX 870M has 3GB of 'memory'.

But Can You Run It claims:

Video Card 1 NVIDIA GeForce GTX 870M
Manufacturer NVIDIA
Chipset GeForce GTX 870M
Dedicated Memory 6.2 GB
Total Memory 4.0 GB

So who do I believe? And what is the difference between 'Dedicated Memory (6.2GB)', and 'Total Memory (4.0GB)'?

I know that MSI supposedly added an additional 3GB to some of the GTX 870's in certain models of the GT70.

Thanks for any help! :)