I am mixed. Yes, I get the question and I know it's because of my hair. Cause otherwise I definitely look like a person of color...and they are usually looking directly at my hair when they ask. And I know it's because they don't expect a black person to have looser curls and a soft look to their hair in it's natural state...so they assume that I MUST be mixed with something other than black cause how else could I have this hair... silly, nu ? LOL!
I think what's happening with you as a white person is that a sterotype may be being placed on you because of what your hair looks like.. people generally associate tighter curls with people of color..specifically black people.
So they probably think your hair is a give away that you are mixed with black which is a big misconception considering the fact that the majority of the world actually does have curly hair it's just in certain countries straightening the hair is so popular you can never tell who has curly or kinky hair...and white women that actually do have very tight curl patterns are rarely seen because they almost always straighten it. So when someone actually see a white person that wears their natural hair in it's normal very curly state..they feel the need to justify it some how..so they assume.. "Oh she must be part black."
And btw, plenty of people of color have freckles and light eyes and well anyone can dye their hair so those are not traits exclusive to white people so that alone wouldn't neccessarily make people think you should be excluded from being mixed.
So I think it's probably some silly ignorant assumption they make about you based on your hair.
I have to admit..it made me giggle that you put "I'm white" twice. I felt like saying.. "Hey .. hey..I believe ya!"