Because this is about trying to get as many folks as possible to BELIEVE their lies BEFORE 480 is released, so that as many as possible will BUY NVIDIA cards BEFORE 480 launches.
ONCE 480 releases, the truth of 480's performance/stock/OC-ability/etc will be known, and this fud and its associated lies evident to all.
Couple this with (this rumor) :
https://semiaccurate.com/2016/06/24/serious-manufacturing-problem-hits-nvidia-consumer-gpus/
And this makes it imperative (from nVidia's perpective) to get folks to purchase as many nVidia cards as possible BEFORE the 480 Release, and BEFORE these rumored supply issues--IF TRUE--become visible and known to the GPU buying masses. Taken together, this would also explain why nVidia pulled forward (by ~1.5 to 2 MONTHS) the 1070/1080 "launch" and the (pure cash grab) of the Founders Edition pricing as well.
If 480/470/460 cards perform and OC well and stock is plentiful, and the rumored (nVidia) supply issues play out, AMD will gain significant market share, and thus nVidia's income stream and market share will suffer.