Spelling with Chinese characters is a tricky business. It is not enough to write the syllables; for each of ~1200 syllables there are several characters to choose from. Which to use in each word is purely a product of history, but the rules people learn are only for Mandarin.

It is funny that Chinese people who only know Mandarin mostly don't know this; it is not safe to talk about in China. So, Cantonese speakers let them continue to believe that Cantonese is "just pronounced differently", rather than being a whole language of its own. (Are French and Spanish just dialects of Latin?)