I suspect you are right.
Well, let's put it another way - we are charged by surface area and the cause is
supposed to be because of the glaze (although a cynical person might just put it down to another pricing model plucked from the air). If I create a hollow ceramic model with the required size escape hole and I
want it glazed on the inside can it be guaranteed that the glaze will properly cover the inside surface? I think not.
So, if I want to create a ceramic version of my Heron (non-frog eating) I do not want it glazed on the inside (although if it is to whatever standard I don't care) and I certainly don't want to pay for that glaze. And I can't make it solid because it is too thick.
And, btw, I have asked this question before on the forums and in mails to service, but never received an official reply.