think it's more that the chapters are pretty long. tho the first volume just had extra long chapters (min 50 pages, ignoring spreads)
checking the raws, they're at 62, and so we can guestimate that, if extended to 14 volumes, that's averaging 4.4 chapters p volume, and say there's about 180 pages per volume (ignoring .5 chapters), then that's a 40 page average, which seems about right
numbers are funky cause i don't know when volume 13 ended and the page count is just an observation, some volumes have more or less that page count (including bonuses usually tho)
if there were more chapters, then that's more chapters per volume
if there were less pages per volume, then that's less pages per chapter