Units sold through would be my guess. This was at the end of the montage right before Layden came out on stage and said PS4 is the fastest selling console of all time.
That's only where they cut off the video at the beginning, there was no sign that that's the final figure they'd reached by PSX. PS4 had shipped 13.5 million by the end of September, there's no way retailers would be ordering that many if they weren't going to sell that number at least by the end of October.
IMO VgChartz is under tracked, which makes sense because of a number of factors, like VG doesn't even have PS4 at 10M when Sony announced they'd sold well in excess of 10 million back in August. When Driveclub launched there were two other major releases, but the sales dropped compared to the previous week when Shadow of Mordor launched, SoM was a pretty big seller, was still on the markey, yet despite there being 4 new games on the market and a highly anticipated 1st party exclusive the numbers still dropped like a stone.
COD AW sold a million more on PS4 than XBox One, according to VGChartz, but the rise in hardware sales was nowhere near as big as Xbox One, which makes no sense.
17M seems like a minimum at this point, obviously sales hadn't reached some milestone that Sony felt they wanted to share, my guess is that they now want to wait until they hit 20M, as it has more of an impact from a marketing POV than coming out and announcing figures every 5 million units sold.
By the end of this year I think PS4 will have sold 20M at least, if not 21-22M, but I guess we'll see.