That’s another huge issue with these “statistics” - apples to oranges would be a generous description...
Their statistics seem to be grab bag of various years and quarters, whichever ones show that particular maker in the best light. For example the RCA tablet's best quarter ever was the 4th quarter of 2014 when they were closing out all their Android tablets at a huge discount and reached for a very short time 4.8% market share and the flared out. The 10% market share for Amazon figure comes from 4th quarter 2015. I could not find where they got the Apple 21.6% and the only way they can get that is to use every toy Android tablet made by every white box maker, whether it was shipped or went directly to landfill. Foot note #1 "Computer repair share percentages and computer reliability scores include tablets" seems to imply the data includes both computers and tablets, but that makes no sense with the data presented as Amazon and RCA manufacture no computers (there is an Rca Computer Sales and Service in Naga City, Philippines).
So it's more Apple, Oranges, plus Bananas, Potatoes, Tin Cans and Cats and Dogs.
I can't figure out how many of which product in what market they are comparing.