Why does "How come?" mean "Why?"

Hi G.W. Holidayers,

Recently (最近), one of my students asked me about the meaning of the phrase "How come?" I explained (説明した) that it means "Why?" and gave the following examples:

     How come you were absent last week?
   （なぜあなたは先週欠席したのか。）
     How come Alan walked here today?
   （なぜ今日アランが歩いて来たのか。）
     How come she doesn't like natto?
   （なぜ彼女は納得が好きではないのか。）

In all three sentences, "How come" can be changed to "Why" in the following ways:

     Why were you absent last week?
     Why did Alan walk here today?
     Why doesn't she like natto?

Please notice that a verb (動詞) follows "Why" in all three examples, whereas a noun (名詞) always follows "How come."

"How come?" can also be used to replace the positive question (肯定疑問文) "Why?" or the negative question (否定疑問文) "Why not?" For example (例えば):

     A: E-Style is closed during Golden Week.
       （G.W.中E-Styleは閉まっています。）
     B: Why? / How come?（なぜですか。）
   
     A: E-Style is not open during Golden Week.
       （G.W.中E-Styleは開いていません。）
     B: Why not? / How come?（なぜですか。）

By the way (ところで), it seems that "How come?" is actually a contracted form of the questions "How does it come that〜?" (どうして〜ということになるのか。) and "How did it come about?" (どうしてそのようなことが起きたのですか。) .

Have a great Golden Week,

Alan