How many people have tried to make a princess line from a block only to find out that no matter how much they try and smooth their curves, the side panel curve is always shorter than the center panel? Before you say, but Stuart you should do it to the side of the BP, you'd be right ... this is a demo illustration only and I don't want to complicate things.
Indeed the side panel is always shorter and you'll see wovens people saying just to ease them into each other so the front panel stands a little prouder. That's all well and good, but as you move further from the BP the amount of difference increases quickly, often to more than can be comfortably eased in.
But in Stretch we can't do that or we end up with tension differences across the seam line ... we have to get our seams to match perfectly.
Some will walk the side panel slowly up the seam to see how much difference there is and then edit the armhole line, but that'd potentially mess up the armhole shape and take away/add width to the upper chest. Instead the way it's done is to stick in a little dart as illustrated below and rotate the side panel until the lengths match perfectly ... it's that easy (especially in CAD when you can automatically measure curve lengths).
Whose head just exploded?