It's an abbreviation... there's different models, so a 767-200 is slightly different than a 767-300, just like 737-100, 200, 300, 400, 500, 600, 700, 800, 900 or 777-200, 300 or 747-100, 200, 300, 400 etc. It's simply easier to write 7XX rather than 7X7-X00.
