I think the reason for the inclusion of the rule regarding letting people in above you in the set, must have been some former dispute upon which some poor MC had to adjudicate.It was obviously a practise that could lead to contention, probably for the reason Robbin proposed, i.e. that you were displacing those in the set lower than you.

Hwever, we do have to bear in mind that the novel was written well before 1813,and probably set in the latter years of the 18th century:therefore, there is a posssiblity that this is one of the anachromisms JA left in the text,and to which she referred in the advertisment she wrote:

ADVERTISEMENT BY THE AUTHORESS, TO NORTHANGER ABBEY

THIS little work was finished in the year 1803, and intended for immediate publication. It was disposed of to a bookseller, it was even advertised, and why the business proceeded no farther, the author has never been able to learn. That any bookseller should think it worth-while to purchase what he did not think it worth-while to publish seems extraordinary. But with this, neither the author nor the public have any other concern than as some observation is necessary upon those parts of the work which thirteen years have made comparatively obsolete. The public are entreated to bear in mind that thirteen years have passed since it was finished, many more since it was begun, and that during that period, places, manners, books, and opinions have undergone considerable changes.

I'll keep looking for the 1798 rules......;-)