One of my students asked this rather fundamental question last week. Good question, and I hadn’t thought about it before, and I need your help to answer it.
BDAG says that the name Ἰακώβ (יַעֲקֹב) is “the un-Grecized form of the OT, is reserved for formal writing, and esp. for the patriarch. It is also spelled Ἰακούβ.
The Greek lexical form Ἰάκωβος, with an alternate spelling Ἰάκουβος, is the Hellenized form of Ἰακώβ.