Just for shits and giggles, I Googled this and found a possible explanation in "Yahoo Answers:
"Margaret Chabris, in the PR department at the 711 headquarters in Texas, adds: “I just received a reply about your question from the son of a former president who was the son of the company’s founder:
A graphics designer altered the “n” during the updating of the logo. In the 1960s, the ELEVEN part of the brand name had been capitalized and at a slight angle through the 7. When the Eleven was straightened out, the designer recommended that the ‘n’ be lowercase for smoothness of the graphic.”