cracked it I think.
just looked at RG13/3552
Wigan - district 1
The enumerators page has the folio no. 2, the first list page 1 has 5, page 3 has 6 etc. etc. until the final page 43 has 26. then comes district 2
the enumerators page has 27, page 1 of the list has 30, page 3 has 31 etc. until the last page 49 has 54.
District 3 enumerators page has 55, page 1 of the list has 58, page 3 has 59 etc.
What I think this shows is that the folio numbers were added at the time all the district returns had been collated for that Parliamentary Borough.
The page numbers re-start with each district.
EDIT:- Ahhh! Then they go and partially ruin that idea when they get to District 8 it has the ref RG13/3553 and the renumbering starts again, maybe it is a second book for Wigan, which does have 51 Districts so they must have numerous books hence the piece no. change.
So the folio numbers refer to each leaf within each book which covers several districts and each book is given a piece number.
That is my supposition/theory.