And now for another aha moment consider this. Just because the ease is different for different fits, the person's neck size does not change. Nor does their shoulder width, their arm length, or their shoulder to waist measurement. Those all stay the same no matter how you want to vary the sweater's body and sleeve widths. So now take a closer look at the schematic for this sample sweater:
Notice that the measurements that don't change show as only one measurement on the schematic while the measurements that depend on the ease all show four sizes. This means the pattern is custom-sized for a single person in four different fits, each with its own chest circumference, arm circumference, armhole depth, and set-in sleeve recess.
Odds are you have never seen a pattern like this before! In fact if you wanted to use a standard pattern but increase the ease for a looser look, you had to go up a size, increasing the chest by four whole inches in most published patterns. Maybe that was just fine, but you also got a wider neck, longer sleeves, and a longer body. If you wanted it to fit properly you had to adjust the pattern yourself or just live with extra long sleeves, etc.
Now that you understand what this is about, I hope you are as excited as I am about having custom-fit patterns with various eases, all for the same person.