That's not exactly the case. 270 miles was the EPA rated range of the S85D. When Tesla released the S90D, the EPA range tests were not complete so they continued to display S85D's 270 mi number for the S90D as well. Then a few months later S90D range on the website changed to 288 mi EPA. A few months after that, it changed to 294. You can see the S85D and S90D range numbers on the EPA website
here under the small car icons.
S90D = 288 miles EPA rated range in February 2016 (
source)
S90D = 294 miles EPA rated range in July 2016 and later (
source)