Rol is saying that if units on shelves/transit are the same as one year ago, then shipped numbers since then are also sold through numbers.
Simple example: A system has 15m shipped and 14m sold thorugh at x date. One year later it has 25m shipped and 24m sold though. The 1m gap between shipped and sold remained the same, so it both shipped and sold 10m units during that year.
Oh ok now it's clear!