Bless mew
Cats express their smarts differently from dogs. They have a good idea of their surroundings and can even pick up a few words of speech. The OP’s cat had somehow taught itself to meow twice (bless you) whenever the owner would sneeze. Out of habit, the owner would utter a “thank you” to the cat, and that was that.
However, whenever the owner would forget to utter her thanks, the cat would constantly meow at them until they got the appreciation they deserved. We don’t know about you, but that is one annoyingly adorable cat. We wonder what other common phrases that cat picked up.