The reason we have double parshiyos in the first place is in order to satisfy the four basic rules (given in Shulchan Aruch, Orach Chaim 428:4) about the distribution of parshiyos throughout the year. In particular, the first two are: that the Shabbos before Pesach has to be Parshas Tzav in a regular year, or Metzora in a leap year; and that the Shabbos before Shavuos has to be Parshas Bamidbar. Since Tzav is the 25th parshah in the Torah, and Bamidbar is the 34th, and since in most regular years there are only six Shabbasos available between Pesach and Shavuos, then the standard layout is that there are three pairs of double parshiyos during Iyar in a regular year.

R. Yissachar ibn Susan (Tikkun Yissachar) writes, then, that if the people of Eretz Yisrael were to split apart an earlier pair of parshiyos (Tazria-Metzora or Acharei-Kedoshim), then that would imply that they are less important than the Jews living outside the Land (in that they have to change their usual practice for the sake of the chutzniks). Instead, then, they wait until the last available pair (in this case, Behar-Bechukosai) and then split that pair up, because at that point there's no choice - they have to do so in order that Bamidbar be read on the Shabbos before Shavuos.

In a leap year, the people of the diaspora keep Matos-Masei combined to get back in sync because in that kind of year (as in most others) it's normal for Matos-Mas'ei to be combined. (Maharit (2:4) says that we don't combine a pair of parshiyos before Shavuos so that the Jews in the Diaspora (who have the possibility of doing so) can read Parshas Bamidbar in its proper place just before Shavuos, and once we're already pushing it off, we try to place the double parshah as close to the next of the four checkpoints as possible, in order to demonstrate that we are doing so in order to keep to the rules. This could even be why Matos-Mas'ei is chosen in general to be doubled instead of Chukas-Balak in years where we need a double parshah to guarantee proper placement of Parshas Devarim before Tisha B'av.)