If by "higher frequency" you mean X rays or gamma rays, sure. That's way higher frequency than you're going to see in PSU EMC testing though.Most ATX cases are made from steel, most higher freq. radiation goes right through it.
Unfortunately I can't find images showing all metals and frequency shielding capabilities.
The Tomshardware test I looked at measured conducted EMC. This means they measure signals travelling along power cable, they're not even looking at radiated emissions. It looks like your other EMI testing link is doing the same based on the bandwidth they're looking at (<30 MHZ).So the noise that was picked up in TomsHardware EMC tests, is emitted by the power cord/mains?
http://www.lessemf.com/probe.htmlWhat exactly is this probe you have, can you link to it?