Question #1 on Parshat Noach

The following question on Parshat Noach is from the commentary of the Abravanel on the Torah, (R’ Don Yitzchak Abravanel, 1437-1508):

Already at the end of Parshat Breishit we are told that Noach was 500 years old when Shem, Cham and Yafeth were born to him, ( 5:32),

וַיְהִי-נֹחַ, בֶּן-חֲמֵשׁ מֵאוֹת שָׁנָה; וַיּוֹלֶד נֹחַ, אֶת-שֵׁם אֶת-חָם וְאֶת-יָפֶת

Why then are we told this for a second time at the beginning of Parshat Noach, ( 6:9-10):

אֵלֶּה, תּוֹלְדֹת נֹחַ--נֹחַ אִישׁ צַדִּיק תָּמִים הָיָה, בְּדֹרֹתָיו: אֶת-הָאֱלֹהִים, הִתְהַלֶּךְ-נֹחַ. וַיּוֹלֶד נֹחַ, שְׁלֹשָׁהבָנִים--אֶת-שֵׁם, אֶת- חָם וְאֶת-יָפֶת

The Ramban explains that the lineage of Noach is repeated, as an indication that these were his only sons. Unlike Noach’s ancestors, who bore sons and daughters other than those mentioned explicitly in the Torah, Noach bore only these sons and no other children. The Abravanel, however, is not satisfied by this explanation; He points out that the Torah explicitly states over there ",וַיּוֹלֶד בָּנִים, וּבָנוֹת " indicating that they had other children as well. Since no such phrase is found by Noach, argues the Abravanel, there must be another reason why his three sons are mentioned a second time.

Can you think of a reason why???