When does a Pokemons memory update in Gen 6

[JLSgolden]

Titel basically. I've read the whole website on Bulbapedia about memories but it doesn't have the answer. Question is simple:

Does a Pokemons memory update every single time I do a memorable action in X and Y? Say 6 Pokemon of mine have different memories each, will every single one of them change to "remembers that it battled at trainers side and defeated the gym leader" if I use them to defeat the gym leader or is it basically just a percentage chance so some will get it and some wont? 

Would really appreciate an answer