I've got a couple of capsules (bought separately, serials are miles apart) which are about 5dB apart too- I just don't use them in a stereo pair. I've requested a matched pair before and been told AKG don't offer the service. From the measurement plots both capsules were within 1dB though and the serials were about 20 or so apart.
I've also got some bought separately, with very different serials which do match (to my ears and meters) although haven't seen what they're like on proper measurement equipment