why引导定语从句,先行词只有一个,就是The reason
如：
The reason why he was late was that he got up late this morning.
而why引导同位语从句,一般只在have no idea之类的句式中出现.
如：
I have no idea why he was late.
其实,在这种句式中have no idea=don't know的意思.
所以,有些语法书上并不把这类从句算作同位语从句的.