Could anyone shed some light on the vast disparity between these figures and the ones that come from Nielsen Soundscan?

These graphs come from the RIAA data which have always tracked shipments, not sales (except, presumably, for digital) from the music companies themselves.

Nielsen Soundscan tracks data from actual sales, and shows sales of billions less than the RIAA data. These tracked sales have been growing steadily for the past 10 years.

Could the RIAA data include shipments outside the US? Could shipments be so wildly out-of-sync with actual sales? How could the distribution channels be stuffed with so many times more merchandise than is actually consumed?

Soundscan here: http://www.nielsen.com/us/en/industries/media-entertainment/...