I was under the impressioned shipped numbers are more accurate then VG chartz, since VG chartz adjusts it's sales after shipped numbers come out
VGC adjustments never exceed 1 million units, and they tend to happen during unexpected changes in the market. PS3 saw an adjustment post Move launch because Move pushed consoles a lot more then expected, and the same happened for 360.
If you really want extremely accurate data, wait for MS, Sony and Nintendo's Quarterly Press conferences. Last one was in September/October, new one happened a couple weeks ago, hence the adjustment to VGC data.