Then that's bad television on the show's part which is what leads me to believe these are not the same Y-wings to attack the Death Star, not that it really matters. If they are then they're extremely important to the larger story, and exist as a connection between Rebels and ANH. In that sense if they're important then as a director or writer you should make sure to show or mention all of them returning.
Don't get me wrong I love the episode, this isnt even a criticism on my part, its excellently done but this may be a small slip up.
Edited by Forresto