"That difference likely comes up because IDC does not count tablets as computers,"

Well there is your problem right there.........

PC = Personal Computer that doesn’t mean a specific operating system or type of hardware, so laptop, desktops, work stations, tablets and honestly even smart phones ARE ALL PERSONAL COMPUTERS.

Most home users only need a computer for email and to use the internet …..considering you can do both and more on any tablet or smart phone these days, there is no reason to buy any other hardware.

If you’re not going to include all devices in these sales figures then they are completely pointless and don’t reflect the reality of the market in 2015. This isn’t the 1990’s anymore where customers might upgrade their hardware on a regular 2-3 years cycle or for gamers (yearly cycle with
constant upgrades)