The original Nexus 7 was one of the best selling Android tablets of all time, and according to Google, it represented a full 10% of all Android tablets sold to date. The early estimates for the new Nexus 7 aren't as aggressive, because the Android tablet market is much more mature than it was just a year ago...Source: http://www.phonearena.com/news/New-Nexus-7-estimated-to-ship-3.5-million-units-this-year_id46151
