Avraham Sacks asks:
Why would the Torah count the years of Yaakov through Yishmael at all, rather then mention it explicitly or through someone else?
Avraham Sacks, Ramat Beit Shemesh
The Kollel replies:
Although there is certainly a reason as to why the Torah chooses to teach us the years of Yakov via Yishmael, this is the sort of question to which there is no end, because one can then question every Limud in the Torah: Why does the Torah teach us this directly, or via a Kal va'Chomer, or via a Hekesh, or via a hint. To explain the matter further, the Torah is written on many levels (based on "Pardes": (Peshat, Remez, Derush, and Sod), and we do not know on which level the Torah teaches different things in different ways.
B'Virchas Kol Tuv,
Eliezer Chrysler