It's still the result of stereotypes and over-generalization. Islam is a religion with over a billion followers: trying to make any sort of statement about all of its followers is a fool's errand. I'm reminded of what my religious studies professor said: "There is no such thing as Christianity: there are Christianities". The Christianity of Pope Francis is not the Christianity of the Westboro Baptist Church, and the same holds true for Muslims.

It's an important disclaimer that race is ultimately a human construct so when if say that something is a race I mean that people either identify themselves or a class of people as belonging to that race.

Back to the point, certain religions are so ingrained culturally and politically that people start to identify themselves culturally and ethnically with the faith because of the people's shared ethical values, history and background. This would have also happened with Christianity if it weren't for the fact that it's the 'default' in the western world.

A person who grows up in the Muslim world, is taught Islamic history, is raised with Islamic values, and practices national and ethnic 'rituals'^1, but is nonreligious might choose to identify themselves as racially Muslim but not religiously.

Conversely a light skinned person in predominately Christian area who finds Islam later in life might say they are racially white, American, or identify with their ancestors ethnicity, but identify themselves as religiously Muslim.

^1 Despite the connotation this isn't necessarily religious. The 4th of July celebration in the US is a ritual.