Well noticed.  People in the Cantonese-speaking area used to write in Classical Chinese for formal purposes and Colloquial Cantonese in less formal communications.  

The use of Written Mandarin had not been attested before the "New Culture Movement', and it soon took over the classical Chinese in many non-Mandarin speaking area. The new written langauge, pronounced in the local vernaculars in the South, was taught alongside the Classical language.  In Hong Kong, where most people had no knowledge in Mandarin, most people from that generation writes in a style that is essentially a mixture of Mandarin and Classical Chinese, with occational Cnatonese borrowings. 

I didn't speak Mandarin before 18, but I manage to write written Mandarin without much difficulty. I think written Mandarin is easier than classical Chinese