I am not familar with this polling methodology so I am not sure what is the cause of this gap
1. Is methodology flawed? For example not representative group selected. 
2. Is it biased? Who is in charge of polls? Does he have any interest in particular results?
3. Are people affraid to show their actual preferences?
4. Other reasons.
5. All obove. 

But this looks like shit. Here where a live it is usually 2-3pp difference.