PDF page sizing in complex, there is nothing that is an actual page size in pdf. Every page can define its own size via several "boxes", see for example:
https://www.prepressure.com/pdf/basics/page-boxes
To understand what --pdf-odd-even-offset does, run:
ebook-convert demo.epub .pdf --paper-size=a4 --pdf-odd-even-offset=36
pdfinfo -f 2 -l 4 -box demo.pdf (starting from 2 as the first page is a cover)
This will show you the boxes for the first three pages. You will see that the
mediabox, which is the largest box is the same for all three pages and is A4 size as expected. The rest fo the boxes are adjusted to give you the offset effect.