After reading it a few times my understanding is this: An H1B job holder applies for a green card. OP then must interview to prove the role can't be filled by

After reading it a few times my understanding is this: An H1B job holder applies for a green card. OP then must interview to prove the role can't be filled by a citizen. An interviewee knocks it out of the park, failing the check and so the green card application is denied. However the person holding the job is still legally allowed to work for 2-3 years in their H1B. So they're kept on for that long even though the check failed for the green card.