A manuscript description may count gatherings while an edition counts chapters. Those numbers organise different things, so they should not be treated as alternative ways of naming the same division.
Follow the fold
The Harvard manuscript glossary describes a quire as folded sheets gathered and sewn together. Several gatherings can form a codex. This physical construction need not align with the divisions of the text copied onto it.
For a harmless paper model, fold two clean sheets together and number the resulting pages. Notice which pages share a sheet. The exercise illustrates a relationship; it does not reconstruct a particular ancient binding.
Read the catalogue before proposing a missing passage
A damaged gathering can raise questions about lost leaves, but a reader needs the specific collation and textual evidence before estimating what is absent. Do not turn a general model into a claim about a manuscript you have not examined.
Keep two outlines in your notes if needed: the physical structure described by the catalogue and the literary structure supplied by the edition. Their relationship can become an interesting research question precisely because they are not guaranteed to match.