"Due to the way in which thermal information is reported on Intel processors, the temperature may be off by exactly +/-15 degrees C." man 4 cpu http://man.openbsd.org/cpu#BUGS
This must be my case. Thanks for participating to Stefan Hagen and John aka j. Topic closed. Sadly, temperature output is not accompanied by warning of known bugs. That would add clarity and save time for users, e.g. it was not obvious to me that man 4 cpu http://man.openbsd.org/cpu#BUGS needs to be read. Although that would add complexity to the code and take time from developers, sadly.
