I get it. Honestly, men don't.
If he's being selfish about other things, then to you, this is the icing on the cake. You may think he is doing it on purpose, and that there is some meaning behind it, but, there's not. He just doesn't see the significance in his choice of words, and you are reading too much into it, because you are very aware.
My husband's family is like this, and at first I thought it was very strange, but now I just ignore it. They will be talking together, brother and sister, and one of them will say "my mom" even though they have the same mom... like it's the same as saying "my car" or something! It's not the way that I or my side of the family speak, but I know that I cannot change them and I wouldn't dream of trying to correct them.
You have expressed your irritation to your husband, and he has tried to change his habits, albeit only somewhat successfully. That's a good sign! He heard what you said and made a change. Now, maybe you could work on the underlying problem of the other selfish habits he has, and you wouldn't be so bothered by his choice of vocabulary.