Thank you for obliging me to delete at the content that could not be shared, in the process of which I discovered the problem, though I still don't understand it. I was removing bit by bit around a couple of tables and found that, when I deleted paragraphs with comments, the ruler-displacement resolved itself. The problem results from having comments displayed in the right margin; when markup display is changed to no markup, the problem disappears and the table is once again adjustable in the usual way. In this case, the comment was in a paragraph just several above the table and on the same page. Somehow, the table, though not straight text, was not adapting to the extra width created by the comments space to the right.
Live and learn. Again, thanks for the prompt.