Here's why:
* "An" is extremely short: It's unlikely to be a complete last name on its own. It could be a nickname, a shortened form of a longer name, or even a misspelling.
* Italian last names are diverse: They can be based on location, profession, ancestor names, and many other factors.
To determine if "an" is an Italian last name, you'd need more information. Do you have a full name, or any other details about its origin?