In most real world applications this isn't the case. Most of the time you will see the frame rates at the lowest resolutions being almost identical, until the slope finally begins. This is due to the limitation of 'the platform' as discussed above. At low resolutions the 3D-chip is stalled because it is able to process data faster than it is delivered by the platform. This effect gets less as resolution increases, which is one reason why the slope is usually starting slowly.
The next thing you might have seen in the schematic fill rate chart above is that I kept the frame rate scores at 32-bit color at the same level as at 16-bit color. This might appear strange to you, because you would never see this behavior in real world applications. In fact, from the 3D-chip point of view, rendering of frames in 32-bit color is pretty much the same as rendering a frame in 16-bit color. As long as the rendering engine is able to handle 32-bit wide data, something that e.g. is not the case of 3dfx's Voodoo3 chip, the pixels can be rendered in exactly the same amount of time. Thus, as long as an application should only be limited by pure fill rate, the frame rates at 32-bit color should be the same as at 16-bit color. Don't ever forget that!