The defence itself has improved in my opinion, and this may he unpopular to some, but for me a big part to that has been Per's organisation since he has come to Arsenal.
What lets us down a lot, (and incidentally makes our CBs look weak sauce all the time) is that we don't defend as a team.
Too often you see midfielders and wingers fail to track back, fail to prevent momentum by opposing team counter attacks and they find it so easy to make yardage in our half quickly because of that.
It makes our defenders look like mugs time and time again. Like it did John Terry under Villas Boas. Everyone thought he had lost it, but back came Mourinho and lo and behold with Matic and a competent team of players giving some kind of defensive shape and he was good as new.
Our defensive record might be good in league seasons because 90% of the time we play teams who are too shit to do anything of these advantages Arsenal give them. But when a team does cone round who have the belief and the knowhow to exploit this weakpoint then we get pasted.
(See Monaco 2015, Everton 2014, Chelsea 2014, Liverpool 2014, 2016, Stoke 2014, Barca 2016, Bayern 2015, Man United 2016, Man City 2013 etc etc)