ASUS released the Strix GTX 950 DC2OC at an Entry / Mainstream price of $159.99 and previous to the GTX 950 NVIDIA was stuck at the $200 price point with the GTX 960. The GTX 960 $200 price point left them vulnerable to sub $200 AMD video cards covering that price point and after an appropriate period of time the GTX 950 was born to fill the gap in the NVIDIA pricing structure. The ASUS Strix GTX 950 DC2OC utilities the same GM206 core that the GTX 960 uses but NVIDIA trimmed away a little of the hardware to make the $160 price range.
Naturally NVIDIA wants to cover all price points so they did a snip and clip on the GM206 Maxwell core and trimmed 256 Cuda cores off the GTX 960 leaving 768 Shaders on the GTX 950. You still have the same 2GB GDDR5 running across a 128-bit bus and 32 ROPS but GTX 960 gets 85 TMUs while GTX 950 gets 64 and those are really the hardware trade offs NVIDIA had to do to field a $160 video card. So short-winded recap the GTX 950 loses 256 Cuda cores and 21 TMUs than NVIDIA turned down the core speed and the memory speed a little and out of the chefs kitchen emerges the GTX 950.