The default behavior for most mobile chipsets is to reduce power consumption when not running on AC. In the past, the technology has been called various names such as SpeedStep (Intel) and PowerNow! (AMD), but the end result is the same. So it's not so much that they "literally have more power" on AC, but rather that mobile parts will clock down to a lower voltage/speed when not on AC to extend battery life.
Hm, that's odd, I actually had a performance decrease when I plugged it in (went from 10000 down to 9600). I chalked it up to the 32 bit not being as accurate. Still loads higher than my sister's brand new XPS, and that's what counts. 😀