To clarify how they arrived at this figure, Rubin clarified that they only count each device once and if it connects to a wireless service provider. He didn't mention anything about devices that only connect through Wi-Fi or have been running on multiple custom ROMs.
Still, whichever way you look at it, it's still a big number and more importantly it's still growing. And although they did not managed to hit the 1 million mark that some predicted they would by October it seems it won't be long before they do.