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As for why they have done it, you could ask the same question about why they have overclocked the 780M and called it the 880M.
You also have AMD to blame for renaming their HD7970M AGAIN. Nvidia already had the fastest mobile GPU so like you say they just needed something new to market to people.
It is a shame that its a little stagnant at the minute. I think the focus went away from the laptop/mobile GPU for a while as the tablets became huge. Then people realised you could only play angry birds or PvZ.