Hello Carsten,
Here's some usefull information about the MAXCPU configuration that might answer your question -
The database parameter MaxCPUs (MAXCPU) is used to notify the database kernel of the number of CPUs from the threads that can be used by the database. However, this configuration affects only UKTs that contain user tasks. Other threads access an unlimited number of CPUs, even if the value of MaxCPUs (MAXCPU) is reduced.
If you are a SAP Customer, more information can be found in SAP Note - 936058 - Question 12
12. Which factors determine how the MaxCPUs (MAXCPU) database parameter is set?
Best Regards,
Yashwanth