Why are you complaining when everybody's given you correct answers?
12.48V is an error of 4%, and ATX allows 5%. So if your reading is accurate, things are fine, unless you paid for a PSU that guarantees tighter tolerances, like a PC Power & Cooling or Antec TruePower.
It's important whether the 12.48V reading came from a digital voltage meter or the mobo hardware because the latter is often inaccurate by 2-3%, and one person saw an error of almost 7%.
Any cheap digital meter is fine, unless you're trying to verify a PSU that's guaranteed to be accurate to within 1%, in which case you want something accurate to better than 0.25% -- or for less money, just buy another PSU.