Many curvy women find that the sleeves of sweaters don't fit. One of the major finds for me when I was writing the Need A Plus Cardigan book was that bust measurement does not necessarily relate to arm measurements. Irregardless, the designer has to still come up with a set of sizes based on bust measurement. Lots of times the sleeves are too big.
If you measure your sweaters, including store-bought double knit jackets you wear, you will probably find that you do wear a large range of sleeve sizes. But you are probably trying to knit your new sweater with an inch to two inches of ease around the sleeve. The schematic is indicating that for your size you are going to get a certain size of sleeve. If you need the sleeve to be smaller, stop increasing on the sleeve about an inch from the bottom of the yoke (one inch less stitches on the sleeve than the pattern says you need at the bottom of the yoke). But what will happen? Won't it distort the raglan line. Yes it will.
I just happen to be knitting a sweater with this feature. I've stopped increasing on the sleeve. Here is a photo of one of the back raglan lines. Can you see where it changes?
Here it is with the black line indicating where the raglan line would have continued and the red line showing the deviation.
Here is a close up.
Doesn't this look remarkably like what your set-in sleeve armhole would look like at this juncture?