[jmull]

Edit: I see others have addressed this in the very long time it took me to express myself!

As I understand it, as batteries degrade it's not only the total capacity that drops, but also the voltage the battery can deliver and the CPU requires a certain voltage to run at a higher frequency.

So, a fully charged, yet degraded battery, may have the overall energy needed to run a CPU at normal Mhz, but may not be able to deliver that energy fast enough.