Hizb 20 closes Juz 10 while still well inside At-Tawba, whose material continues into Hizb 21 without any sura boundary intervening at all, one of several stretches in the middle third of the Quran where a single long Medinan sura carries a reciter across an entire hizb, or more, without the relief of a chapter change that several earlier hizb divisions in this account have shown crossing back and forth between suras instead.
In tarawih recitation this hizb typically closes the tenth night, and its four rub al-hizb quarters trace a path from warnings about hoarded wealth, through calendar and expedition material, to the cave narrative, and finally to the excuse making of hypocrites, four distinct subjects held together only by their shared sura rather than by any progression the hizb boundaries themselves mark out.
For huffaz, the cave narrative's comparative brevity within this hizb, a handful of verses inside a much longer legal and historical stretch, makes it a commonly used anchor point for locating one's place within the quarter, since it is more easily recalled by content than the surrounding, more abstract material on wealth and excuses.
That same absence of relief recurs later in the thirty-juz scheme wherever a single Medinan sura runs to unusual length, and At-Tawba, spanning parts of four separate hizb divisions before it finishes, is one of the clearer examples of it.