Are you sure you are measuring on the same place at your body?I did. And what's odd is the jacket that fits me well seems to measure 21" pit to pit and yet when I measure my chest I'm ~42.5".
Makes no sense to me.
The armhole on this jacket looks like a regular mid to low armhole. If you put this jacket on you, notice where the lower edge of the armholes are on your body. Then you measure your chest between those points and do you still have a reading of 42.5"?
Typically, when one measure one's chest, he will measure it just under the armpits, for the largest size. That is fine. However, the pit-to-pit measurement of a tailored jacket does not equate to that point. Unless the armhole is custom-made to your body just under your arm, that point under the armhole of the jacket will be lower than the place where you measure your chest.
Therefore, I would suggest you to put on this jacket and mark where the lowest point of the armhole is at your chest. Measure your chest around that size and see if that number is larger than 21"x2. If that number is indeed larger than 42", there must be some magic with this jacket that can wrap around a body larger than it is measured, or the measurement of 21" is incorrect, which means there could be some extra fabric at the back that is not shown.
You might as well post a picture of you with this jacket on to show how fit this jacket on you. There might be fit issue, but I have no clue until seeing some pictures.