A standardized set of twelve gods could be thought of as a ‘representation of the pantheon’, which is common to all cities. Thus, it is an integrative feature, almost a Panhellenic one. If so, you might think that the members of Dodekatheon would always be the same, but in fact the members varied like everything else in Greek religion. The most often cited group is: Zeus, Hera, Poseidon, Demeter, Apollo, Artemis, Ares, Aphrodite, Hermes, Athena, Hephaistos and Hestia. Some are more secure in their position than others. Hestia and Ares tend to be the first to be jettisoned and we may find Dionysos, Herakles or Hades sneaking in. But the variations are often even greater. One strikingly aberrant list is the Olympic one, known from Herodorus, where the deities were arranged in six pairs: Zeus and Poseidon, Hera and Athena, Hermes and Apollo, the Charites and Dionysos, Artemis and the river Alpheios, Kronos and Rhea. ... The Olympian Dodekatheon is the best attested variant pantheon, but there may have been others. At Chalkedon, the twelve established by the Argonauts are reported by Herodorus (again) to have included Hades, apparently replacing Ares. Hades–Plouton was also included in Plato’s Laws. There may be another variant Dodekatheon at Pherai in Thessaly, where Stephan Miller reconstructed an altar with the heads of six named goddesses, which are plausibly taken as the female side of a Dodekatheon, and which include Einodia (a form of Artemis?) and also Themis. - Ian Rutherford, Canonizing the Pantheon: The Dodekatheon in Greek Religion and its Origins
Discussions and works based on Greek myth are often so rigid about which gods should or shouldn't belong to the dodekatheon, given the variations that existed in Antiquity. It looks as if the only debatable place is that of Hestia or Dionysos, but the idea of the six altars at Olympia and the altar of the six goddesses at Pherai indicate that Aphrodite, Demeter, Artemis or Hera could also be absent from the Twelve, not just Hestia. Dionysos could be excluded, but so could Ares and Hephaistos. It is also misleading when people speak of the gods as the "twelve Olympians" with the implication that "Olympians" are necessarily a select group of only 12 gods (when for instance Herakles also could be worshipped as one of the Olympians as attested in Herodotos, Histories 2.44 and the Muses are called Olympian by Hesiod), or that the twelve must necessarily live on Olympos. It's the 12 gods, not the 12 Olympians.
