The thing about messing up measurements comes from rushing or misinterpreted body shapes .
They try to match the most important measurement to a existing size and pattern and then modify accordingly.
My all weather halfbelt has a 43-44 shoulder and my cossack has a 47-47 shoulder even though I specified the same measurement.
They took a existing pattern and modified it .
The stock had straight shoulders and after complaining about the first version I wanted round ones .They probably had a fixed formula for round shoulders like 3cm upward from point XYZ.
So they moved that point upward.
The stock pattern had a size XYZ neck opening which moved 3cm upward.
So now they connected the dots and didn't think about it too much .
Then they add the custom sleeve opening and after going back to a fixed formula they again connected the dots .
It is really easy to overlook certain areas if you work on a tight schedule or in a certain speed.
A premade properly scaled pattern is way easier.
They don't start from scratch probably.They match and modify .
If size M has a 24cm armholes and size XL has the desired 26cm armholes and your base pattern is size M then they take the XL sleeve and modify from there. If the XL sleeve is matching the desired bicep shape then they take that shape and size .If the cuff you want matches size M relatively well then they take that and add it to the XL sleeve .
It is mix and match to make it even possible.
However this can lead to problems.
Slimming XYZ down changes things from curved to straight for example.
They try to match the most important measurement to a existing size and pattern and then modify accordingly.
My all weather halfbelt has a 43-44 shoulder and my cossack has a 47-47 shoulder even though I specified the same measurement.
They took a existing pattern and modified it .
The stock had straight shoulders and after complaining about the first version I wanted round ones .They probably had a fixed formula for round shoulders like 3cm upward from point XYZ.
So they moved that point upward.
The stock pattern had a size XYZ neck opening which moved 3cm upward.
So now they connected the dots and didn't think about it too much .
Then they add the custom sleeve opening and after going back to a fixed formula they again connected the dots .
It is really easy to overlook certain areas if you work on a tight schedule or in a certain speed.
A premade properly scaled pattern is way easier.
They don't start from scratch probably.They match and modify .
If size M has a 24cm armholes and size XL has the desired 26cm armholes and your base pattern is size M then they take the XL sleeve and modify from there. If the XL sleeve is matching the desired bicep shape then they take that shape and size .If the cuff you want matches size M relatively well then they take that and add it to the XL sleeve .
It is mix and match to make it even possible.
However this can lead to problems.
Slimming XYZ down changes things from curved to straight for example.