Why do a lot of Americans think that Spaniards aren't white?

I am entirely of Spanish ancestry from Galicia (northwest tip Spain), I have very pale skin and have dirty blonde hair and yet I have encountered many, many people who think that I am a Native American! I mean, not that there's anything wrong with being Native American like many, per say, Mexicans or Nicaraguans are but people think that I am just trying to imilate to America when I say that I am white.. Why do many Americans think that the Spaniards are not white... Also, Galicia resisted the multiple attempts of the Moors to conquer their land, so I do not have Arab features....