Anemone:

Its a different voice dub person for each main actor is my understanding- and then other roles can sometimes be the same voice dub person but with changes in tone. 

AvenueX - on youtube explains it well.

https://www.youtube.com/watch?v=q2wAzbgjt8A - why same voice actors are used for main roles over and over and over again

https://www.youtube.com/watch?v=GcRYGmzcWv8&t=665s - why they dud in the first place

China dubs their dramas and movie mainly because there are many different dialects and accents. They might not know how to accurately pronounce the words or their accent might accidentally come out when they're acting. But when they dub they can try as many times as they want to get it perfect. As to why sometime the person that acts is different from the person that dubs, I'm not really sure. Maybe they just don't like the way they speak or their voice or perhaps they just can't get the accent right? I'm not really sure on that aspect