Uncanny Valley: Sunday gives this vibe even to his admirers — mostly because the man is so damned BIG. Chesterton's narration describes him as being out of scale with his surroundings, like a statue carved slightly larger than life-size.
Syme finds the whole Council to have this effect, but more so in the case of Sunday.
Most of the Council of Days has something about them that is slightly off. Combined with Syme's general overimaginativeness, this makes them seem almost inhuman. This is, of course, more or less intentional in order to keep their disguises.