You are all wrong actually. It is one singular, continuous series with multiple arcs, the
Kase-san Series.
Everything reflects this, including official materials, retailers, Seven Seas, Wikipedia. No-one in Japan thinks of this as multiple serieses. All of the chapters being on one page is simply correct.
As for the issue of the chapter numbering, many serieses have inconsistencies between their magazine chapter numbering and their volume chapter numbering. In general, MangaDex prefers to follow the volume numbering. The thing is: the Kase-san Series completely removes chapter numbers for its volumes, as well as moving some chapters around (including punting 2 to a doujinshi). What you have to understand is: the magazine chapter numbering
was never going to be respected because it does not align with the volumes. So, once you're looking at the chapter-number-less volume chapters, it only makes sense to apply MangaDex's default preference of continuous chapter numbering. That's why the page is how it is.
@Valuz could contribute by tagging the volumes at the very least, if not aligning the numbering, but they choose not to, so others have to.