I think it has to do with what kind of ghost they are. Are they a living ghost, a memory ghost, a ghost that broke through the veil or escaped the sea of the damned, etc…
Like Ramsey is a living ghost, supposedly given eternal life by a necklace he wears. The Ghost of Graymarrow is a summoned ghost, that can only exist after performing a ritual. Briggsy appears for the SoSS as a memory ghost, as it is her human form and not her final form(skeleton lord) that is seen. Phantoms and ghost ships were brought to the SoT through a rift between the Sea of the Damned and the Sea of Thieves.
I do agree it’s a bit confusing and probably overly complicated, especially for anyone who is new or didn’t follow the lore. Like Briggsy’s whole story was spread out so much over time that it’s hard to follow what her true ending was.