The 800 series already exists as a series of mobile GPUs (800M). They're about to be replaced by the 900M series of mobile GPUs. Nvidia thinks (or is rumoured to think) that it doesn't make sense to launch a flagship series of desktop GPUs with the name of an older generation. Therefore, they skip the 800 name and go straight to 900 by rebadging the 800-series of desktop GPUs that's in development to 900.