It is a bit strange, isn't it? But I think I remember them doing it before with a new release; releasing a driver that only works with the specific card. Either way, give the 337.88 driver a go; it should hopefully resolve the problem of it not detecting your 770.
EDIT: just double checked, and yep, the nVidia site lists the Titan Z as the only supported product for the 337.91 driver.