It's because the media is a piece of fucking garbage and take practically every goddamn thing that's said by anyone out of context. Oh, and the folks who run Slashdot do effectively zero checking on anything they post. /rant
Now the explanation: Prior to the answer Musk gave indicating that Teslas would do 1000km on a charge, he was talking about a recently set record where a dude (Casey Spencer) did 500 miles (~800km) in a Tesla Model S, driving at something like 24mph for like 24 hours. In that context, Musk said that similarly, a 1000km could be achieved in a Tesla by 2017, given battery density improvements of 5-10% annually. All that would be necessary would be a 20% improvement on the record by 2017. I might add that the dude who did this was in a 85kWh car going downhill for a decent portion of the drive and took into account weather effects, temps and whatnot to achieve his 500 miles. I wouldn't be surprised if the latest 90kWh Model S as is could do another 100 miles if tightly controlled in the right conditions (high altitude, ideal temp/wind), so really a 5% improvement in both 2016 and 2017 is all that's really being predicted here.