I don't think the old Formats mentioned circled item numbers either. The only reference to that seems to be in that example. With the lack of a specific rule or even a comment, my own judgment is that's the way someone thought it should be done. What is the purpose of these shapes? Do they add to the meaning or are they simply a visual or decorative device to draw visual attention to the number of the item? Note that in several places in tne Formats, there is commentary about transcriber judgments, especially with regard to the omission or inclusion of font attributes.
Please send a print page and I'll be glad to take a closer look.