有句:“who served you the man standing behind the counter.”

问题描述:

有句:“who served you the man standing behind the counter.”
我想,如果用关系代词是用who,但是应该是“who served you the man who is standing behind the counter.”为什么省略了who与is?

一个是定语从句
一个是现在分词做后置定语
由意思上也可以看作是省略,但并不是所有情况都能省略