When the contour line around a glyph is open, as for instance at the neck in Ca6-28, it is a sign meaning 'not of this world' - i.e. 'on the other side of the horizon', 'in Hiva' - which implies the being in question is a 'ghost', 'spirit', 'aku-aku':

The pu glyphs have closed contour lines - there is no indication of 'Hiva'.

The glyphs maybe should be imagined as if turned around with the hole in the east at left and the hole in the west at right:

When the hole in the east is opened to let in the sun, the opening is still a phenomenon in this world of ours: