Seems that Sanders is either making up ground very quickly or this poll is biased towards him.
Assuming this is still basically an internet-exclusive poll (irrespective of the refinements of the methodology), it likely favors Sanders to some degree. However, I tend to also believe that the PPP poll is favoring Clinton to a similar degree. Presidential primaries are largely nationalized affairs at this point and it makes absolutely no sense for Sanders to be receiving anywhere from 30-40% of the white vote nationally and in most states, but only 10-20% in SC (hopefully everyone will save the "but muh southern whites don't like Bernie" nonsense).
As regards to the SC poll that might be true. However when you look at the New Hampshire poll, that also has Sanders much higher than in other polls on RCP. The three most recent there are low 40s. The trend is for Sanders to be improving but the size of this jump seems unlikely. It is quite possible he could reach there in another month though. The polling in Iowa has been a bit more inconsistent in other polls but this poll for Iowa is also better for Sanders than any poll on RCP.
All three together indicate that there is probably a bit of a bias towards Sanders in these polls. However they might be replicated soon if Sanders continues to improve his position in the polls as he has been lately.