Shoulder shaping on BACK side (cardigan)

I am working on the Patons book 1318 “Cardigan with eyelet pattern”.

According the the pattern, I have to knit 3 separate pieces: 1 piece bodice and 2 sleeves.

I am shaping the shoulder on the BACK side and it’s near to the end, it says:

“Cast off 9 sts at beg of the next 4 rows, then 8 sts at beg of foll 2 rows. Cast off rem 29 sts.”

In my understanding, the shape of the back shoulder should look like an isosceles trapezoid /_____
According to pattern, if I cast off 9sts, knit to the end, purl back, cast of 9 sts again, then the shoulder will look like a right trapezoid |____\

Please clarify. Thanks a bunch.