Stump Your Rabbi

We blow the shofar after ne’ila on Yom Kippur.
Even if it’s Shabbos, before nightfall.

When Rosh Hashana is on Shabbos, we don’t shofar at all.

Why shofar the Shabbos for a minhag but not for a mitzva d’Oraysa?

Shabbos Shalom
& G’mar Chasima Tova