"How come?" is an idiom meaning "how is it that?" or "why?" You say this when you want to know the reason for something. Examples: How come you're so late?[ Why are you so late ?] How come you have changed your hairstyle?[ Why did you change your hairstyle?] How come they did not come to the party?[ Why didn't they come to the party?] How come he forgot to pay his speeding fine?[ How is it that/why did he forget to pay his speeding fine?]

"How come" = Why? They are pretty much interchangeable.

How come: If you want to show disbelief or surprise about an action, you can ask a question using 'how come'. example : How come he got the job? (You can't believe that they gave the job to somebody like him) How come yelling on a mountain can start an avalanche?