It's his 13th Masters title compared to Federer's 24th, using any meaningful comparison. To suggest otherwise is like saying Celtic have only won the league three times because the SPFL was only formed three years ago.
Yes, The 915 points being mentioned is for the 2016 ATP race, not the rolling 12 month official rankings. Djokovic also has 1000 points to defend in Paris, so they could go into the Tour Finals practically neck and neck.