I actually think 25.5 million is too high for the XBO. Believe it or not, we can figure out a ballpark figure for the XBO's current worldwide sales. EA revealed that combined console sales were 55 million by the end of 2015. Sony announced that the PS4 sold through 35.9 million as of January 3, 2016. Through simple subtraction, the XBO sold about 19.1 million by the end of 2015. According to NPD, the XBO sold 11.12 million by the end of 2015, meaning the US constituted about 58.2% of the console's worldwide sales. As of the end of November, it sold 14.34 million units in the US. Divide that figure by 0.582, worldwide sales are estimated to 24.62 million.
However, this calculation hinges on the assumption that the US percentage remained the same from January to now. I think the US is currently taking up a larger share of the XBO's worldwide sales than before. According to Marketwatch, FY17 Q1 (July through September) hardware revenue decreased by 26% in comparison to FY16 Q1 due to lower prices and lower volumes of consoles sold. However, the XBO was up YOY by about 99.7K units on NPD for that quarter, leading me to believe it is down YOY in the ROTW. Being up YOY in the US and down YOY in the ROTW would increase the percentage of sales taken up by the US. Let's say the US makes up 60% of the XBO's sales. 14.34 million divided by 0.6 equals to 23.9 million, which is even lower than VGC's 25.5 million estimation.