I think the logic is that you, the Adventurer, are given the distinctive @ as "that's where you're at" (or some other, better, way of phrasing it). And you know what you are (dwarf, human, animal man, etc). Companions are something I've not perfected the recruitment of (always used to be more trouble than benefit, those you could even recruit, and I'm not quick to change on that front), but if they become @s too then that's to show party-membership I presume, to override examples of their cousins' appearance that they'd otherwise take.
When running the Fortress, you don't have 'a' dwarf (and the idea is to see them as independent pawns, not directly possessed extensions to your will), so faces (or bearded faces, depending upon the exact tileset rendered) are chosen.
Depictions of 'others' then rotate around that. Not the 'you or yours' character, but the global raw-native one or (when it clashes) gives trader-dwarves the '@'dventurer symbol to distinguish them from your faces.
(I'll have to fire up a fort to check this... I'm just so used to seeing and understanding whatever I see that I'm not entirely sure what I do see... It just makes sense when I do.)