Why do so many people think that I'm mentally retarded when I tell them that I graduated college with a 2.5?

Some of my friends stopped talking to me immediately after they found out what my GPA was. They used to tell me before that I'm well-rounded and have a lot of interests. What is there to laugh about a 2.5 GPA? I have a good job as a researcher at a neuroscience lab. I wouldn't have been hired if I was really retarded.