Why do some people think Catholicism is different to Christianity?

I am a Catholic, and I often see Catholicism being treated as different to Christianity. For example, on websites where you can give your religion, Christianity and Catholicism are often two separate options. This irritates and slightly insults me. Isn't it obvious that Catholicism believes in Jesus, and that is the definition of Christianity? Not only this, but Catholicism was one of, if not the first branch of Christianity (I am not sure whether Cahtholicism and Orthodox were one and then separated a few centuries A.D. Could someone help me with this too please?) Why do people think this?