What am I missing?

I am interested in getting a covert coat, but I cannot wrap my head around how they are sized. According to the Cordings web site, "Covert Coats are designed to be worn over jackets so buy them in your normal size for the optimum fit." It is my understanding that US and UK suit sizes are the same. A 42 R, which is what I am,commonly comes off the rack with a chest of 22"-23", shoulders in the 19" to 20" range, and sleeves around 25." The Cordings covert in 42R has a chest of 23.5" and sleeves of 28.25." Most other models, both UK and US are similar in measurement: generally a slightly larger chest than a 42R jacket (but not by much), more narrow shoulders than a 42R jacket, and sleeves 2" to 3" longer than a 42R jacket. What fine print somewhere did I miss?

What am I missing?