They just can't speak in proper mandarin very well.
Just like you can read English well, but it doesn't mean you can speak it as smooth as Americans.
Some people have a very strong local accent that makes their mandarin sound very weird. Many of overseas Chinese can't speak mandarin properly as well, although they speak mandarin almost everyday. These people in entire community don't know where is the wrong part, they speak it like it's an OK, because all their neighbor and entire city speak wrong too.
Simply speaking, just like the different between Indian English, Singaporean English, UK English, US English, etc. Local accent is very strong that...many people hard to understand each other speech.
In my opinion, it's Taiwanese English that is the most correct and easiest to be understand from ALL of THEM...just kidding.
