This happens to me all the time. I'll be standing around in a store, and strangers will come up to me and ask for help. I'll be walking down the street, and someone will ask for directions. I'll be part of a group doing a tour or something, and they'll go right past the actual group leader to ask ME what we want to see.
I guess it's a leadership gift of some kind; either that, or just natural arrogance. But it's weird. For an introvert, I don't fade into the background very well.