No idea how they make up sales numbers.
Apple's own sales numbers say they sold 74 million iPads in 2014. Not sure how gartner lost 4 million.
Also, Apple's numbers are reported as sales to users, everyone else uses sales to channel (the channel can return unsold stock to the company in the following quarter but can still claim it sold that many)
They can't claim in their financial statements shipments to sales channels as sales if a right to return clause exists. If companies claim sales on shipments to channels when a right to return exists then they are committing fraud (see Worldcom and FASB Issue No. 48). Also, adjusting Apple's units sold to 74 million doesn't change Apple's market share by much. By my calculation, Apple market share goes from approximately 38% to 40% and Google goes from 62% to 60%. Hardly a big change.