The spec for the -12 V rail (and -5 V rail, if used) is actually +/- 10%. The positive rails require +/- 5%. You are well within the spec.
Also, as a general disclaimer: in order to have useful measurements you need to measure with a multimeter or similar (not the motherboard's onboard reporting available via software), and ideally measure under load.
http://www.formfactors.org/developer\specs\atx2_2.PDF