The former refers to the cases when to a syncretised form corresponds an individual being or, in other words, it has a singular content and identity. A different case would be the latter, i.e., functional syncretism, which is when two or more deities are combined in one syncretised form for a practical reason. Consider the case of Hermes and Anubis, for instance: they’re both openers of ways, guides, and guardians, so if a worshiper wished to address to both of Them on something both gods share as function, one option is to use a common, syncretised form. Something like, say, Hermanubis. It’s not same as saying that They’re the same god, for in this perspective They are two different deities, but who are called upon collectively for specific functions They share. It’s a bit like two people sharing a phone number for a business they’re both in and people using it instead of calling to each individually for the same job. And there are more possible examples from the godly realm: Mercury and Bonus Eventus, both granters of luck and successful outcome; Mercury and Janus, guardians of the doorway; Neptune and Njord, gods of the sea or just the coastline and great river mouths.
Of course, I’m not saying that this is how the pre-Christians saw syncretised deities. This is rather a personal take on the issue by a 21st century polytheistic who felt challenged by the issue, both from outside the community, where sometimes the question arises in the form of critique to ancient polytheisms (empty rituals for invented gods), and within the pagan community, where there’s a monistic trend that uses pre-Christian syncretism to support its modern claim that all gods derive from a single source. For someone who uses ancient polytheism as a basis for modern practices, reconstructionist or recon-derived, this poses a challenge that I tried to address.