In my workplace, it's common to speak informally, but we have a lot of people who work/study with us who are from countries where showing respect through titles is paramount. So I often get a [my title] [my real first name] combination too. I've always taken it to mean "I respect your desire to be called by your first name, but I caaaaaan't just leave the title off!"

For what it's worth, I don't correct them either. :)

-J