The reason for this is that Pb4-55 is a special variant of 'niu' (GD18) with a kind of henua as lower part, whereas the parallel glyphs in H and Q are more normal in appearance:

    

They have the normal type of bottom, leading the thoughts towards toko te ragi (GD32). The glyphs in H and Q have haga (GD36) at right, not marama (GD44). Though the shapes are indeed not so dissimilar.

The addition of marama after Pb4-55 presumably indicates that this henua is a period of the night, not of the day. That it is lightly bent is equivalent in meaning - I guess - to the addition of haga in the parallel niu-glyphs in H and Q.