천일문에서 궁금한건데요

565. Having been jobless for six months, he could not pay the bills.
= Since he had been jobless for six months, he could not pay the bills.

두 문장이 동일한 뜻을 가지고 있다고 나와있는데...
Since he has been 이 될 수없는 이유가 주절의 시제가 could 라는 과거이기 때문인가요???궁금해요~