The answer to your question may be contained in the question itself. When making your original conversion between the folio/page reference and the folio recto/verso-style reference, you said:
For example, RG 9/2480, f 70, p 11 would become RG 9/2480, f 70v.
See Wikipedia on Recto and Verso for Left-to-right language books (such as English):
Page 11 cannot be a verso page if the language is English.
I plugged the reference you provided into Find My Past so I could browse the images. (A disclaimer: it is not safe to assume that the order an online provider serves up images is the same as they appear on the microfilm -- the pages could have been re-arranged by the vendor. For an example, see Elizabeth Shown Mills' QuickLesson 16: Speculation, Hypothesis, Interpretation & Proof, footnote 10.)
If you start at the front of the piece and page forward, it quickly becomes clear that the folio number only appears on the recto page. The page numbers appear to be printed on the pages, while the folio numbers were added later (probably stamped) -- they are likely to be a safeguard against missing pages. But the folio numbers are also needed because the printed page numbers are not exclusive within a piece (piece 2480 has more than one "page 1" in it).
It may be that the convention came into being because recto and verso have different meanings, depending on the language one is accustomed to, and don't have the same context once an item is microfilmed. Perhaps people felt that adding the explicit page numbers to the reference would make the citation more clear, especially since the folio numbers do not appear on every image.