The birth order of the sons.
The Ohr Hachaim answers his fourth question – why the Torah tell us the final count of three sons when the number is obvious? He quotes a Gemara in Sanhedrin (69b) that the children here are enumerated in the order of wisdom, not birth. Shem was the youngest son, Yeffess was the oldest, and Cham was the middle son. The Gemara deduces this from the possuk that says (Bereishis 11:10) "אֵלֶּה תוֹלְדוֹת שֵׁם שֵׁם בֶּן מְאַת שָׁׁנָׁה וַיּוֹלֶּד אֶּת אַרְ פַכְשָׁׁד שְׁנָׁתַיִם אַחַר הַמַבוּל" - These are the descendants of Shem. Shem was one hundred years old, and he fathered Arpachshad two years after the Flood. Noach was six hundred years old at the start of the Mabul, one hundred years after Shem was born, so Shem must have been 102 years old. If he is considered 100 years old, he was obviously not the first to be born.
If the Torah would have written וַיּוֹלֶּד נֹחַ אֶּת שֵׁם אֶּת חָׁם וְאֶּת יָׁפֶּת, we would have thought that this was the birth order, and Shem was the first to be born. Now that the Torah says that he gave birth to three sons, the birth order is not mentioned, and nobody will make this mistake.
