Oh and to give a proper answer to this 'why', Well, I live here but my Chinese isn't great (yet). What I have noticed, and is widely accepted in fact, is that Cantonese (Guangdong/HK) sounds remarkably like Vietnamese, particularly the south, due to their shared history, borrowed words etc. 60% of Vietnamese are borrowed from Chinese.

If I'm not listening out specifically for what's being said, I can easily mistake Cantonese for Vietnamese in exactly the same way I can mistake Italian and Spanish if I'm not focussing, too.

Thankfully, as I said, this is not even relevant because it's still an absolutely true occurrence in China