Okay, now that is something that I completely don't understand? I can't tell you why it is that way but what I can tell you is that you're very lucky. Running at lower voltages is better for the CPU since it'll produce less heat! Now if you have a 366 that runs at 550 MHz on 1.85 Volts than stop there!
Pushing it to 578 MHz will make a very small difference and the PCI and AGP slots don't like it very much.
I say stick to what you already have!
It Is Now Safe To Turn Off Your Computer!
? Quantum Physics ? Light travels faster than sound. This is why some people appear bright until you hear them speak.