It has something to do with the mobile 800 series. Some GPU's still uses the kepler architecture.
So they thought it would make sense to skip the 800 series for desktop graphic cards because they are all gonna be Maxwell.
It's odd that the 820M, 830M, 840M, 850M and 860M are using the new Maxwell but the 870M and 880M are using the old Kepler.
I looked into it and found a good explanation from http://www.kitguru.net/components/gr...mid-september/
Model numbers of modern central processing units and graphics processing units have been criticized criticized for years since in many cases they barely reflect performance and positioning of products and in many cases cannot even determine their generation. At present Nvidia offers high-end GeForce GTX 700-series of products for desktop computers. At the same time, the company sells GeForce GTX 800M family of products for notebooks. What is rather confusing is that both lineups – the GTX 700 and the GTX 800M – are based on similar chips powered by the Kepler architecture.
In a bid to avoid confusion with its upcoming families of graphics processing units for desktop and mobile computers, Nvidia decided to sell its products powered by the second-generation Maxwell GPUs code-named GM200, GM204 and other under the GeForce 900 moniker, reports VideoCardz.