Why do Americans view Columbus and Spanish explorers as white, but think of modern Spaniards as mexicans?

Some people just don't think, and substitute their experience for using their brain. Heck, we all do that from time to time. But for people in the USA, meeting a Spanish speaking person usual means they're from Mexico, or at least from Latin America. So, they begin to automatically ume: Oh, Spanish speaking=Mexican. When they meet someone who speaks Spanish (or Castillan to be more accurate) who doesn't fit their expectation of what a Mexican should look like, they find it confusing. For many Americans, however, especially those who have gotten around and been educated to the ways of the world, we do understand the difference, and understand that not only do the Spanish not look like your "typical" Mestizo (one of mixed heritage--both Spanish [or Portuguese] and Native American), residents of Latin America often do not fit this stereotype. Hope that helps.