Is this one of those power supplies that keeps the Earth ground and computer ground separate? So when it's operated outside the case, those grounds aren't connected together, but when bolted to the case, those grounds connect to each other through the metal case and several of the standoffs connected to the motherboard's ground.
Separate DC and Earth grounds isn't that unusual. I have old Delta power supplies built with the grounds kept separate, and some early Antec SmartPowers had a soldered jumper to connect the grounds on their circuit board. About every AC adapter with a 3-wire power cord will have separated grounds.