The reason for this is because you have enabled PowerNOW!.
PowerNOW! is a power-saving feature that is implemented into all mobile AMD-CPUs.
It will dynamically adjust both clockspeed(via multiplier change) and Vcore depending on CPU-usage...i.e. if your not doing anything, the CPU will run with reduced clockspeed and voltage, thus saving power - if the CPU is being used(e.g. when playing a game) it will automatically switch back to full speed and voltage.
More info here
If you're using WinXP, PowerNOW! is controlled via Power-schemes, i.e. if you select 'Always on', your CPU will run at full speed all the time whereas if you select 'Laptop/Portable', PowerNOW! will be dynamically adjusting both clockspeed and voltage depending on CPU-utilization.
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