I actually understand the difference between tube and pipe measurments. I've just never been a fan of fractions in written sentences as it can lead to confusion. Really, just never been a fan of fractions. Not blaming you for US standards of measurment.

In plumbing, you find a 2" pipe and a 2" elbow. I would have assumed it translated the same in fencing so that a 2" post has a matching 2" fitting. It doesn't matter if 2" represents ID or OD, but within the same industry I would assume all measurements were applied the same.