Ohr hayashar to hilkhot mamrim 6:3 points this out, but doesn't seem to answer.

Daf Al Hadaf to Pesahim 56a quotes several answers. The sefer merafsin igra (page 233) is cited as suggesting that in reality the sons added honorifics to his name that are omitted by the verse.

Alternatively, it suggests on the basis of berakhot 4a that an inherently flattering name may be used without appellations. The name Yisrael is flattering, alluding to his overcoming various forces.