Memes aside, there's a simple reason for the page discrepancies (which is the same reason this series could accumulate 51 chapters in only 2 years and change in a monthly magazine), and that's that each issue of Jump SQ typically included 2 chapters with a combined page length of 19~20 pages (except for the first 9 chapters, which were released in batches of 3).
Most of the chapter pairs last volume were pretty equal in page size, but starting with chapter 18 they really start fluctuating wildly. There's a lot of double sized single chapters from here on out, as well as pairs where one chapter is longer, so the other one ends up with less pages. That's what happened here, chapter 21 was 12 pages, so 22 only got 7. This happens a lot from here on out, and some are even more lopsided (eg, chapter 36 is 15 pages, and chapter 37 only 4 pages)