My 7 yr old dog eats her poop and the Cats poop, why?

Now I've done plenty of research as to why dogs eat feces. It could be stress, mental disorder to hide their "product", malnutrition, or a learned behavior. Now My 7 year old chihuahuawill go into the cats litter box and eat their poop. This scares me because I don't want her ingesting any poop really and the kitty litter. She doesn't do it all the time because she knows its not a good thing so she stopped eating her poop, she used to when she was using the litter (paper litter for dogs). Now she uses a puppy pad and she doesn't eat her poo any more. But I've noticed when me and my husband are not looking ( but paying attention) she will go inside the litter box and start fishing for poopies. I do not get it. I think she thinks it tastes good maybe because its so pungent. Can anyone yze this to where I can understand my dog and why she eats poo for fun?