To answer that there are two things:

1> All multi-frequency processors have a performance curve, where they perform best. For the N900 processor, that curve has been mapped a few times, and every time it's a bell curve with a peek right around 500Mhz. It makes sense that this is the case, since it's sold as a 500Mhz processor. That's what TI optimized everything for. So 500Mhz is the most energy efficient when it comes to doing any real solid number crunching. (Clearly sleep mode, or 0Mhz, is more efficient when it comes to not doing number crunching. )

2> Imagine you have a background process that wakes up twice a minute to check some values. Let's say it runs for a quarter of a second at 500Mhz. That's not enough to trigger the system to go up to the next frequency. It wakes up from 0, runs, and the system goes back to sleep. That same process at 125Mhz would run the processor for up to 4 times longer, and probably trigger a move up to 250Mhz. Lets say it triggers the jump at one half second, the 125Mhz/250Mhz version would take 3/4 of a seconds. (250 would take a half second, where 125 would take 1... Half the work is done in .5 seconds at 125, the other half is done in .25 second at 250) After an hour:

500 Mhz = 30 seconds of run time, 59.5 minutes of sleep time.
125/250 Mhz = 1.5 minutes of run time, 58.5 minutes of sleep time.

After a day? The 500Mhz system has run for 12 minutes, where the 125Mhz system has run for 37 minutes. Which one used less energy, assuming 0 mhz takes close to 0 power (when compared to any running state)? Now add up all the little apps running time slices in the background that wake up on occasion to do work. That translates into a reasonable amount (5% to 10%) of battery savings over a day. That's called the "race to idle" effect.

Between those two, choosing your most effective speed for your base speed actually causes less energy use. Thus why most people here who have good battery life are using a 500-X clocking method, be that 500-600 for non-overclockers, or 500-850/900/1100 for overclockers.

Btw: You may note that overclocking does in fact mean you're sliding down the other side of that bell curve for energy efficiency. The more you overclock, the more your battery drain can be when you're using the device for long-term number crunching!