I'm *guessing* that perhaps they're daemons like SystemIServer or automount or something they'd rather people not mess with. But again, that's just a guess.
I presume you CAN see a PID? Check for the PID and see if the same PID appears in top, then you can see what's being hidden.
top -l5 > topout.txt
That command (that's a -Lfive not a fifteen) will output a log of TOP to a file 5 times, refreshed every 1 second, so you don't have to watch both top and pv furiously. Then you can use cat topout.txt | grep "xxx" replacing xxx with the PID you noticed in processviewer to find out which process is hidden.