The front measurement includes the raglan armhole shaping. You're working the back to 15 inches and then doing the next step, which is the armhole shaping. You'll continue with the armhole shaping on the back until a certain number of stitches remain, which you will bind off. When you do the front, you do it the same as for the back, (15 inches) then doing the armhole shaping but stop when the front measures 22 inches. At that point you proceed with the neck shaping.
For the neck shaping - look at the picture of the sweater. Do you see how the middle part of the neck is lower in the front than the shoulders. What you're doing is binding off those middle stitches while you work both sides to the shoulder, with separate balls of yarn. Later you'll come back and pick up those stitches to knit the neck band. Does that help? If not, holler and I (or someone else here) will try to walk you through whatever part you're stuck on.