Snapchat gave me the real name of a user I dont know. How?

A long time ago I added a person to Snapchat. His username (display name?) was O*, and I added him with that display name. Today I received a message from him, but the name that showed up in my notifications was S* R* (his real name). When I opened the message, his regular display name O* was displayed. What can explain that his real name showed up in my notifications?