I've noticed that most people act like Christina Aguilera isn't Latina, and that's she's purely non-Hispanic (I won't say white here because Hispanics can be of any ethnicity). I thought it was obvious, especially considering her surname is Aguilera, which is obviously Spanish. People get shocked when she sings in Spanish and they're like "but she's white". As previously stated, Hispanics and Latinos can be of any race, not just mestizo (European/Native mix that is dominant in most Latin American countries). Her father was from Ecuador!
Another thing that really shocked me was when Futurama, one of the most intelligently written shows out there, made a joke that compared acting like something you're not to "Christina Aguilera singing in Spanish". *Facepalm*

Its either because they cant tell if she is or not or she has blonde hair and some people think that latinas dont have blonde hair.

I had a similar discussion with a girl from work (years ago now). She would not believe that Mariah Carey had African/American bloodlines.

Christina is more obvious due to her surname though.