Difference between Accrued Revenue and un earned revenue

Contingent expense is  there in accounting parlance. Unearned revenue is generally not provided unless there is a certainty. 

There is nothing called unearned revenue apart from contingency. If it's sales, it's is already recognised. So what other contingency exists? Unearned/ unrealised (both are close here) gains exists on revaluations but not revenues.