Isn’t it more based on the fact that we get more corners than everyone else and concede fewer? I think our %, certainly offensively, has always been fairly shocking.
Id guess you are dead right. People often defend our overall poor set pieces with the argument we have the best record etc but it's really just a numbers thing.
Believe it or not statisticians are able to account for things like...the total number of corners.
Last season City ranked highest in the league for goals conceded per corner, and goals conceded per other set piece, as well as goals scored per corner
It's in the article @Pingu the Penguin posted above.