This could sound VERY STUPID, try using a different meter, the same situation happened to me last week, and it turned out 2 out of 3 meters we had were reading wrong!!!
But if its really the alternator, have you tried getting a price to get it reco'ed.
Also, have u made sure u earthed your system up properly? Any other electrical components u recently did... because it sounds like u have a bad earth somewhere. Hope this helps.