[求助]GWD-5-20 (than usual?)

studies show that a disproportionately large number of the scientists who produce highly creative work beyond the age of forty entered their field at an older age than is usual.

一道逻辑题，但其中出现than is usual

请教这里省略了什么？？ 比较的是什么？

为什么is不省略？？ 不是说as usual  /as anticipated / as estimated 是习惯用法吗？

GWD-5-20

Scientists typically do their most creative work before the age of forty.  It is commonly thought that this happens because aging by itself brings about a loss of creative capacity.  However, studies show that a disproportionately large number of the scientists who produce highly creative work beyond the age of forty entered their field at an older age than is usual.  Since by the age of forty the large majority of scientists have been working in their field for at least fifteen years, the studies’ finding strongly suggests that the real reason why scientists over forty rarely produce highly creative work is not that they have simply aged but rather that they generally have spent too long in a given field.