Because there is more to a graphics card than just memory. There are frequencies, multipliers, ratios, pixel pipelines, shaders, etc. Also, the brand makes a difference in certain games because the numbers of the previously stated things will change.
The graphics card works by changing data into a format the monitor can display. First the computer dictates what the screen should look like (via directx or suchlike). The video card's driver then determines how to tell the card this. The data from the driver is transmitted to the card by PCI, AGP, or PCIe slot. The card then uses it's own CPU (called a GPU) and memory to transform the incoming data into an incredibly fast seiries of pixel discriptions (on/ off/ colour). It does that so fast that it can describe the state of millions of pixels many times per second, giving the illusion of motion. The monitor then simply shoots the designated pixels onto the screen.
For dual GPU's working together, each GPU simply handles half of the pixels on the screen.
Various manufacturers use different GPU's. Some cards have faster memory than others, some have faster GPU's. The bus might be different (PCIe is faster than AGP).