@Random commenter did you read ennead or did you google it? spoilers..
Because Orsis is Anubis's real dad but Nephthys (Seth's wife) cheated on Seth with Orsis and had a child with him (Anubis) but at the time seth didn't know that so he raised Anubis then Orsis r*ped Seth and planted a seed in him and then goddesses of destruction told Seth the truth etc etc...