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why do or why have How come birds fly south in the winter? How come you got invited and I didn't?
Usage notes: usually used to introduce a question, but also used to react to a statement: “I haven't watched a football game all year.” “How come?”
How is it that, as in How come you're not attending the conference? Sometimes how come follows a statement and asks the question "why" or "in what way," as in You're not going? How come? The related phrase how so? functions the same way, as in You say she's changed her mind-how so?How come is short for how did it come about that and dates from the mid-1800s; how so, short for how is it so or how is it that , dates from about 1300.
How is it that; why: How come you're so late?