Ideas:

This type of glyph is always used in Large St Petersburg when the glyph in Large Santiago looks like this:

On the other hand, in Large Santiago both types of glyphs are used with about the same frequency. Consequently the writer of Large Santiago probably distinguished between two different meanings and used two different types of glyphs, while the writer of Large St Petersburg used only one type of glyph for both meanings.

I believe that the straight ends - which always are used in Large St Petersburg - represents the fundamental glyph, while the concave ends are signs, perhaps to show that the time period is belonging to the Moon (the concave ends).