刘老师，您好！ 谢谢答复。 less than they were 中的they were 是否是错误，应删去？ 不然怎么理解这里的they were呢？

可以省略，也可以保留。这是同一个事物的自身比较，比较的是不同时间。即： adoptions in 2020 are adoptions in 2019 were = they were 2019（换成代词后，把2019 放在后面） 又如： Tom is taller in 2020 than (he was) in 2019.