That SS-300ET doesnt seem to have -5v input what I saw the label showing in newegg. maybe the result is false too.
Correct, it doesn't have a -5V. Any PSU that follows ATX revision 1.3 or greater will not have such a line, because it was removed from the ATX spec.
The hardware monitor is probably just confused since the voltage isn't present. Normally you'd think it'd just show 0V in such a situation, but for some reason it's more common for those things to spaz out and show random numbers instead.
It's nothing to worry about, in any case. If you don't have any devices that use the -5V (and you probably don't... or at least none that you'd want to use in a PIII system), then its absence will make no difference whatsoever.