SSS上的一句话 they were actually more likely to make the purchase than were those

SSS上的一句话 they were actually more likely to make the purchase than were those
Subjects who saw ads with warnings were initially less likely to buy the products.But when surveyed again some time later,they were actually more likely to make the purchase than were those who saw ads without the warnings.
they were actually more likely to make the purchase than were those who saw ads without the warnings.这句中的than were those 为什么这么用?直接than those不就行了吗?为什么要加个were?