为什么?

1.两者不一样.
2.no matter who引导的只能是让步状语从句,在句子做状语,对句子其他主要成分如主语/谓语没有任何影响.
如：No matter who is on duty today,he must clean up the classroom.
3.但是whoever有两种理解.
1）whoever = no matter who,同样引导让步状语从句.
如：No matter who is on duty today,he must clean up the classroom.
= Whoever is on duty today,he must clean up the classroom.
2）whoever还可以用来引导名词性从句,包括主语从句/宾语从句/表语从句/同位语从句.
此时,whoever = anybody who,翻译成“任何...的人”.
3）该题恰恰是用whoever引导一个主语从句,做句子的主语.
即：(Whoever leaves the room last) should remember to turn off the light.主语从句
= Anybody (who leaves the room last) should remember to turn off the light.定语从句