Set plays are better because we aren't giving away stupid freekicks. We are actually a good defensive team in open play.
We have conceded less goals from open play than Man Utd and Arsenal, but the most in the league by set pieces. It stands to reason that if we can sort this out, we will be in a very good place next season.
Offensively, we have scored the 7th most from open play, with the only teams ahead of us being the top 6. Not too bad.