960 has a WAY better price/performance ratio, they sell for around $200 and perform good as a mid end gfx card
the 760 on the other hand is over $300 and performs much worse so you are comparing a cheaper card and a more expensive card, the cheaper card performs better and the more expensive card performs worse so that is that and the 960 actually performs around the same as a 770 when it has a Very good overclock