exhaust manifold port size difference

[cadillac_al]

I am fixing an exhaust manifold on my K20 with a 350. I thought I would compare it to another one I have on hand from a 77 C10 with a 305. It is fairly easy to tell with the naked eye that the ports in my K20 manifold are larger than the 1977 manifold that I have. When I measure them they are about .050-.060" difference height and width. I was always under the impression that they were all the same size. The castings and markings are almost identical. Do you suppose some manifolds have smaller ports, like for a 305 or is it just manufacturing tolerance?