Why is Prometheus according to some sources not considered a Titan?

Why is Prometheus according to some soources not considered a Titan? Is the below information correct?

“Forethought.” Is sometimes called a Titan though in reality he did not belong to the Titans, but was only a son of the Titan Iapetus whence he is designated by the patronymic Iapetionides"

That is odd. Prometheus is always considered a Titan in everything I recall. Typically the Titans are the 12 original children of Uranus & Gaea and their children. Though not the 6 children of Cronus & Rhea. (The Olympians).

I wonder if they’re not counting Prometheus as his mother Clymene is a nymph and not another Titan. As far as I know though, Prometheus’s brother Atlas is absolutely considered a Titan. So none of this make much sense.

Perhaps as the great benefactor and possibly creator of humans, he is honorarily considered not a Titan? As far as Greek Gods and Titans go; he is one of the most benevolent.

Here is the Genealogy, it is of course a mess of inbreeding.

I guess in reality the Titans could be said to be the original 12 and Atlas???

It might be a purely generational thing: You could argue that the first generation were the Primordials, the second generation were the Titans, and the third generation were the Gods. So Prometheus, then, would be of the same category of beings as Zeus and Poseidon.

Though I don’t think that’s a very strong argument, given that offspring of two god parents was also a god, like all of the children of Zeus and Hera.

Does Prometheus have any offspring, in Myth?

Deucalion

The Greeks were seldom consistent in their references to creation and genealogy. The Deucalion link, e.g., gives three possible mothers and the Pantheon link the OP gave lists three possible mothers, none of them overlapping.

Stories about the creation of humans are even more widely diverse, with the gods not necessarily involved. Sometimes Prometheus gives fire to humanity and sometimes he creates them first. The versions changed over hundreds of years and evolving traditions; no single organized comprehensive source emerged.

Hesiod’s Theogony is the earliest surviving source on Prometheus. Blame him for the son of a Titan heritage. But who listens to Hesiod anymore?

I’ve also read where Epimetheus and Prometheus (who were brothers) where like Goofus and Gallant, respectively. (Hence their names meaning “afterthought” and “forethought”.) Epimetheus also reportedly married either Pandora (of “box” infamy) or Ephyra (daughter of Oceanus, she was a water or sea nymph depending on the story).

Epimetheus and Pandora had a daughter Pyrrha, who married her cousin Deucalion and had a whole litter of kids (including Hellen, who was the progenitor of the Hellenes, another name for the Greek people themselves). Though other stories say Hellen is the son of Pyrrha and Zeus. Greek myths are definitely not consistent.

There’s also one source that calls Hellen the son of Prometheus himself. Yes, consistency is in short supply in a mythical tradition that starts out mostly-oral, regionally variant and then patchily-transcribed over centuries.