Question about processor speed

I'm sure this question has been asked before, I did a brief search, and couldn't find it.

 

I have an Inspiron 1150. 

 

Why is i that even while plugged in my processor is only running at 1.56GHz?  My processor, as listed below is a 2.66 GHz.

 

This is what the Dell Support Program shows. 

 

Processor: Mobile Intel(R) Pentium(R) 4 CPU 2.66GHz

Processor Speed: 1.56 GHz

 

I know that the CPU speed decreases when the laptop is NOT running on AC, but shouldn't I be getting the full 2.66 GHz when I AM running on AC?