Compared to VGC:
Ofc PS4 still has units in transition and on shelves so i think the 2M difference are pretty fine.
Overall VGC has a tendency to overestimate Playstation hardware. And my guess is that its the same for the Vita which sold so bad that Sony does not want to disclose numbers.
VGC has a tendency to overestimate/underestimate Nintendo, PlayStation, and Xbox hardware (hence why we adjust when official reports are made). :P
Really, I don't think we're that bad, though. Considering we're the only site to do it.