PSN Games = PSN downloadable games, with a scant few titles that may be considered analogous to Games on Demand (RE4HD, Metal Gear Solid Collection, Ratchet series, Jax series). No PS1/2/Vita games. Although I could probably make an estimate for something like VII, which is probably around 600-750k sold.
As for how the data is obtained... That is classified at the moment. Probably will tell later this year.
How did you make that estimate?