I had wondered about DIPM, however my Intel 520 SSD is installed in a desktop machine and I believed that DIPM was disabled.
According to the specification quoted above the recorded value for the power-on hours should therefore match the clock time.
I have just confirmed that DIPM is disabled using the Intel SSD Toolbox and the instructions given in the discussion thread that you referred to - Word 79 Bit 3 is set to zero.
So I am still puzzled...