Fallout 3 runs better on medium settings than low?

I bought a Radeon 6570 HD recently and decided to play Fallout 3. When messing with settings to get a smooth FPS I noticed that the game runs better at Medium settings than low settings, it's much more choppy at a low setting.

So I was just wondering if anyone has any ideas why this happens?

Other Specs Ram 3Gb P4 3.2 GHz HT Motherboard (unsure, one that comes with Dell optiplex gx 620 )

In low settings, it is likely that graphical effects, and algorithms used for them are less demanding, to reduce the use of the GPU.

Because of that, it is possible that the CPU gets an extra pile of calculations in low settings. Mind you, probably much less complicated than the original ones going to the GPU, but still.

In your configuration, since the CPU is the bottleneck (being a bit old), it could explain this special case where the "low" preset is actually slowing your game down.

Now I'm only speculating, and talking about a general case. It might be caused by something else, for Fallout 3.