这个 lower 是表语，其前的 has been 是系动词 be 的完成时态，又如下面句子中的 lower 也是表语，而不是宾语：

A man’s voice is usually lower than a woman’s. 男子的嗓音通常比女子的低。

The number of people who attended was lower than expected. 出席的人数比预期的要少。

These children scored significantly lower on intelligence tests than others in their peer group. 这些孩子在智力测验中所得的分数远远低于同龄群体中的其他孩子。

Our current revenues are certainly lower than anticipated. 我们现在的收益的确比预期的少。

We have a buoyant economy and unemployment is considerably lower than the regional average. 我们经济繁荣，失业率远低于地区平均水平。

The latest survey shows buying plans for homes are sharply lower than in June.  最近的调查表明计划买房的人数与 6 月份相比骤减。