最后一小句，为何逗号后直接跟谓语动词double?省略主语的话无需加上连接词and 吗?

Even as profits slumped, investors in the sp 500 benchmark index earned 14.3% in 2020, about double the typical return over the past 20 years. 

about double the typical return over the past 20 years 

这部分是名词词组，为14.3%的同位语。double在此为倍数词，作return的前位限定词。double在此不是动词。你看看句子谓语动词为过去时态。