if there had not been a hard layer of rock beneath the soil,

if there had not been a hard layer of rock beneath the soil,they would have completed the job in a few hours.这句话为什么要用过去完成时呢?为什么不用一般过去式呢?改成这样 if there were not a hard layer of rock beneath the soil,they would have completed the job in a few hours这样不是更好吗?为什么呢?
用过去完成时他语境更加强调了什么呢？