Given that tp_call is just "ternaryfunc", my guess would be "because when the iterator protocol was added, someone went with function-pointer-type-per-slot rather than function-pointer-type-per-call-signature".

We *are* a little inconsistent here (e.g. "reprfunc" could also just use the "unaryfunc" signature), but Stefan's right that that isn't a good reason to *add* to the inconsistency.