For those of you dealing with sleeves that are either too long or too short on RTW jackets. This picture will help you understand why the less you have to shorten a jacket sleeve, the better off it'll be. This visualizes the problem of the sleeve vent and why you can't make it higher as you shorten a sleeve.
Where it says "old pressing line" is where the original length of the sleeve would be. Those white lines are there because this sleeve was getting lengthened, and as you can see, you can only go so far.
Let's say you needed them shortened by +1". The red line indicating "New pressing line/cuff length" is where it would be shortened to. That small extension of fabric is the vent. That sharp right angle at the "top of sleeve vent" is where the outside sleeve seam travels up the elbow. The only way you can raise a vent is if the maker has a really wide seam allowance on the outside sleeve seam or they just extend the vent way up the sleeve, but there's really no reason for them to do this because it's RTW. You just end up with a really short vent if you need sleeves shortened ALOT.
This concept is identical in jacket vents in the back of the jacket. It's why you normally can't add double vents to a jacket that has no vents at all.