Spanish is one of the best languages for having the spelling match the pronunciation, but it's not perfect. First of all, you can't spell something just based on hearing it because a /k/ sound can be a 'c' or a 'k', and a /s/ can be an 's' or a 'c'. It also has silent letters like 'h'. Going the other way, seeing the spelling of Mexico, Xalapa, Oaxaca, etc. would lead someone who didn't know to try to pronounce them with a /ks/ sound, but they're really pronounced as if they were spelled Mejico, Jalapa and Oajaca. Then, there are loan words like "psicologia" where the "p" is retained from the original language, but not pronounced in Spanish.