IDG are also known as IDC. You may have heard of them.
They are the people to go to for actual figures, estimated figures and projected figures. So much trust can we put in these numbers that even Sony and Microsoft use these reports from IDG in their audited financials, even Take Two are quoting them. IDG use numbers from NPD, Gfk and other well known trackers to come to their totals and charge 10's of thousands of dollars for just one report.
So anyone saying these numbers are not a true representation of sell through are wrong. IDG are telling us the cumulative install base of PS4 and X1 as exceeding 29 million units.
AMD also told us that sell through to end users was nearly 30 million. AMD could very well be using IDG figures.
Therefore we can estimate that sell through will be between 29-30m with over ~18.5m attached to PS4.
VGChartz also charges in the thousands for reports, but their numbers are always funny.
IDG is still an estimate, nothing more. Sell-through is probably more like 26m combined. PS4 at >18.5m and Xbone at 8m.
If I had to guess, AMD was originally talking about shipping to retailers, but realized that this would mean Xbone sold-through less than 10m. Microsoft got very angry and ordered AMD to 'do something about it', so AMD did some poor damage control about how they were talking about sold-through when they said shipped, knowing that both shipped and sold-through figures would still be close to 30m.
User moderated for this post -RavenXtra