I found the answer myself: "My -12V, -5V, etc. show voltages that are way out of specification
The -5V rail was removed from the ATX specification once the ATX12V specification replaced it. Furthermore, a number of motherboards simply do not use either the -5V or -12V rails. Either lack of -5V or the motherboard's lack of utilization of -5V or -12V can cause very erratic readings. These readings should not be of any concern."
Found at: http://www.bfgtech.com/bfgpower/troubleshooting.html