The stats are sales over the last 3 months, not install base. However, Statcounter tries to measure usage, not unique devices. Netmarketshare tries to measure individual browsers/devices.
For example, if you browse 10 sites in a day on an Android phone, but your neighbor browses 990 sites a day on her iPhone, the Statcounter numbers with just you both being counted will be 1% Android and 99% iPhone even though the install base is 50% each.
This is the reason that Statcounter and Netmarketshare browser share numbers are so different from each other, with Statcounter showing Chrome in the lead and Netmarketshare showing IE in the lead.
For example, if you browse 10 sites in a day on an Android phone, but your neighbor browses 990 sites a day on her iPhone, the Statcounter numbers with just you both being counted will be 1% Android and 99% iPhone even though the install base is 50% each.
This is the reason that Statcounter and Netmarketshare browser share numbers are so different from each other, with Statcounter showing Chrome in the lead and Netmarketshare showing IE in the lead.