The first question I would ask is how good is the ohmmeter you're using. The quality of the meter has a lot to do with how accurate a reading you'll get. But in your case I don't believe that there is too much difference between 0.01 and 0.02 ohms. The fluctuation could be caused by the OE coils being properly grounded and the ones at Autozone being set of the parts counter (no ground). I'd be more concerned with the spark plug wire terminals at this point. They should all read about the same. Again, 12.7k ohms isn't too terrible far from from 13k ohms, but if the spec calls for 13-15k, then I would say that your coils are starting to head south. If you have the money I'd replace them. If not, I start saving and be ready to replace them as soon as possible.