Unexpected feature of laptop P4

Boggle.

When I plug the laptop in, the cycle counts go down by about a factor two.
I was assuming that, if the CPU slowed down when the machine was running
on batteries, the RDTSC counter would also slow down and the cycle counts
remain reasonably constant, or go down at the high end because the memory
is still working in nanoseconds rather than cycles.

But it looks as if everything has slowed down by about a factor two
except for RDTSC which keeps on its 2.8GHz counting, and conceivably
as if the L2 cache has slowed down more and decided to be about half
its former size. This is bizarre.

a) Does anyone know why this happens?
b) What does it say about contemporary computers that I haven't *noticed*
   that the machine runs its CPU at half-speed when unplugged?

time            wallpower       battery ratio
128             2.3             4.8     2.1
256             2.3             4.4     2.0
512             2.2             4.6     2.1
1024            2.2             4.5     2.0
2048            2.2             4.4     2.0
4096            2.3             4.6     2.0
8192            2.4             4.4     1.8
16384           20.6            58.3    2.8
32768           21.1            48.6    2.3
65536           20.5            55.5    2.7
131072          20.4            56.3    2.8
262144          19.4            54.3    2.8
524288          212             1136.6  5.4
1048576         885.4           1798.6  2.0
2097152         918             1802.7  2.0
4194304         919             1800.9  2.0
8388608         911.6           1808.7  2.0
16777216        918.8           1815.5  2.0
33554432        921.5           1855.2  2.0
67108864        911.9           1921.1  2.1


Tom