If you're from Georgia, at least my part of the state, when someone asks "Why" often they will say "How Come?" I really have no idea where this originated, however, I can imagine it was between a mother and child. You know the age, when every other word out of a child's mouth is "Why?" I imagine the conversation being something like this:
Mom: "Honey go wash your hands for dinner."
Mom: "Because you should have clean hands when you eat."
Mom: "Because there are germs on your hands and you might get sick."
Mom: "Because germs can make you sick if they get inside your body."
Mom: "I Do Not want to hear the word 'why' come out of your mouth again tonight."
Child: "How come?"
All of the "How come?" wondering came about when I was making dinner tonight I realized I needed to start a new grocery list even though I shopped for groceries last night. Without thinking I said "How come I missed adding mayonaise to my list yesterday?" When I realized what I'd said I had to laugh.
Speaking of mayonaise, if you like Duke's Mayonaise they are celebrating 90 years and offer a free cookbook ($6.50 s&h,) free recipes and a store finder on their web site. It's the only mayo I use.