The Tur (OC 428), as explained by the Tiferes Yisrael (Shevili d'Rakia 15), lists four dates that must fall out at certain points in the leining cycle:
- Pesach must fall out after Tzav in a non-leap year or after Metzora or Acharei in a leap year
- Shavuos must fall out after Bamidbar
- Tishah B'av must fall out after Devarim
- Rosh Hashanah must fall out after Netzavim
In order to account for these, Shvili d'Rakia continues, depending on the year and place, the following parshios might be combined or split:
None of these make sense. Why can't we pick other parshios to combine? For instance, this week is Tazria-Metzora. Why don't we combine Shemini and Tazria instead, in a non-leap year? Why not combine Emor and Behar instead of Behar and Bechukosai? Why not Vayeilech and Haazinu instead of Netzavim and Vayeilech? There are any number of combinations that could be made for each of these gaps; why do we pick these in particular?
(If you want to point to length, we combine Mattos-Maasei and Vayakhel-Pekudei. If you want to point to similar content, we combine Chukas-Balak while we don't combine Vayeilech-Haazinu, or Lech Lecha-Vayeira, or Vayeishev-Miketz, or Va'eira-Bo, or Bo-Beshalach, or Yisro-Mishpatim, or Terumah-Tetzaveh.)