The simplest, unromantic answer? The Founders didn't change any of the GD signs.
A deeper answer, though, is that it's possible to look at these signs in different ways. Meaning isn't intrinsic, it's something we bring to things. There isn't an official teaching in A.'.A.'. on this, so I won't give you one... just invite you to find your own understanding in the signs. One hint (or, rather, something to play with): Consider that there is some pattern wherein these four signs relate to the four quarters of the Sun. Another hint: How does the same myth cycle look from the p.o.v. of some other character besides Osiris.
PS. Thanks for the kind words on Pearl