In what case doesn’t a coupon correspond to a segment?

I read:

If you’re flying AAA-BBB-CCC and then CCC-BBB-AAA, you might expect to have four coupons: AAA-BBB, BBB-CCC, CCC-BBB, and BBB-AAA. A coupon doesn’t necessary correspond to a segment, however. – choster Jun 4 at 19:18

In what case doesn’t a coupon correspond to a segment?