Its sell through, USA is based on NPD, Japan based on Media Create/Famitsu and Europe based on Gfk while RotW are estimates which explains the slight discrepancies.
Those USA numbers are indeed NPD, but i'm telling you those spanish numbers are not sell-through i saw the same GfK data in Spain saying that, so i can't believe any of the other european markets that are probably using the same tracking system. And in the end, the same guy put the total of PS4 and Switch using shipment numbers, so there's something fishy in this data.
This article from Vandal, the very same page that give us updated numbers from software in Spain put this article.
It shows the numbers from 2015 through 2018 and they are what i posted earlier. Between 500-600k each year. And i have to remind you that PS4 had an excellent start during its first year too (ending 2014).
Then this other article citing Sony itself saying PS4 have sold 3'3M lifetime around mid-half 2019 since launch.
That graph you brought says PS4 had already sold 3'6M by the end of 2018. If Spanish numbers are wrong, i can't trust any of the other european numbers showed in there.
Edit: And besides, i think i remember now Zhuge saying in resetera that those numbers were not completely accurate in Europe but they were close enough.Last edited by colafitte - on 02 December 2019