VGC overtracks XB1 by quite a bit.
When EA accidentally revealed the actual figures last time by stating that PS4+XB1 have sold 79 million units:
At the same time Sony had revealed that PS4 had sold through 53.4 million units: https://www.forbes.com/sites/olliebarder/2017/01/05/the-ps4-has-now-sold-53-4-million-units-worldwide-with-6-2-million-sales-over-the-2016-holidays/#756123e7b415
Meaning XB1 at the end of 2016 had sold 79-53.4=25.6 million units. At the end of 2016 VGC has XB1 at 27.9 million!
So even if VGC tracked XB1 properly this year it's still over tracked by 2.3 million, but most likely it's over by a good 3-4 million over tracked.
Without the official numbers, VGC will probably never be able to correct the numbers and the error will just accumulate over time.