Mark,
that is correct, the AKG CK62 has about a 10 dB lower noise level referred to 1 Pa than the B&K 4133 and Gefell MK202.
But, the B&K and Gefell have a much lower (not specified) lower mechanical harmonic distortion. Measuring capsules often run on a much higher polarization voltage allowing for larger distances between backplate and membrane, and also much less smaller backplates than the actual width of the membrane, all to lower harmonic distortion from the capsule itself.
B&K could easily get values like the CK62, or even better with the 4133, by making the backplate and membrane closer together, but then the distortion esp. in the low end will be much higher. One microphone where the backplate and membrane are extremely close, is the Neumann TL(M)50 omni, and as an effect of that it produces a warm ( = louder) lowend, and often problems in humid envirements (nickel capsule).
Erik Sikkema