Other Tesla owners have reported this exact problem (inaccurate charging gauge) in the Roadster; other reviewers have noted this problem in the Model S. Moreoever, this problem is endemic to all pure-electric vehicles in cold climates, such as Nissan Leafs and Chevy Volts. (Google for references)

Ergo, the likelihood of (1) the reporter not fully charging his car's battery and (2) the fuel guage inaccurately reporting a fully charged battery is the realm of more likely than not.