R.Y.
Previous answers about adoption seem to have that aspect covered.
It is not unusual for people to have different last names in the same family. My aunt and uncle have a family of 4 but there are 3 different last names--mom has her maiden name (went back to it after a divorce) her older son has his bio dad's name and her daughter and husband have the same last name. She kept the maiden name in the second marriage so her son would not feel left out. In my family I am kind of the odd one since I never changed my name when I got married. My kids have my name as a middle name and dad's last name. It works fine for us and there are so many non-traditional families these days it is no big deal.