Why would they give it the GTX 880 designation if it isn't the full featured upper end model? So what if the performance cannibalizes the Titan series. Those are old architecture that need to go away.
For the same reason that the GTX 660 used the GK104 chip instead of the full GK110 chip. Nvidia's mid-range GK104 was performance-competetive with AMD"s high-end Tahiti chip found on the HD 7970. Nvidia was able to take their mid-range chip and sell it at high-end prices because it outperformed the competition and would sell at that price. Then while AMD evolved their GCN architecture for the Hawaii chips in the R9 290 series Nvidia was able to sell off their high-end GK110 chips for top dollar as Tesla compute cards and eventually roll those chips into GeForce cards for the 780, 780Ti, Titan, and Titan Black.
My guess for this generation is that it's the same deal. Nvidia feels that their mid-range GM104 chip will be competetive with AMD's offering so they will sell the GM104 as the GTX 880 and hold onto the larger GM110 chips for high-margin Tesla cards and roll them out later as the GTX 900 series.