I would have expected that someone more knowledgeable than me would answer.
After some long digging, I found the solution. Windows 7 reads the information in question out of so called ACPI tables, provided by the BIOS. It then uses them according to its internal policies, but you can change it.
From a cmd.exe with Admin priviliges, do:
powercfg -attributes SUB_PROCESSOR 4d2b0152-7d5c-498b-88e2-34345392a2c5 -ATTRIB_HIDE
After this you suddenly will find a new option in Energy Management under Processor Energy Managment. It will allow you to change the interval the need for a switch up or down is evaluated. On my machine the default was 30 milliseconds. I changed it to 100 milliseconds and voila: I see the machine switch up less often. Before, even on an "idle" system, it switched up from time to time because of some background processes.