Because dyn and ci are masculine nouns but cath is feminine.
Masculine ei causes soft mutation while feminine ei causes aspirate mutation.
lladd mutates softly to ladd but does not mutate aspirately, so it remains ei lladd for feminine ei.
You'd see a difference in a word starting with p- t- c-, e.g. gaeth ci ei brynu; gaeth cath ei phrynu for "a dog was bought; a cat was bought" (to buy = prynu)