Why is that??
Djokovic is 1800 points behind Rafa in the race. He has 405 points to defend, whilst Rafa has 600 points to defend. Rafa may not reach SF in TMS in Shanghai, like last year -> may lose 100 points there(Blake, Gonzo, Djokovic present in the first eight with positive h2h in their meetings), has QF at NY to defend, say he reaches the same. QF in Madrid, very fast HC, say he will reach the semi with 75 points +, will add further 100-200 points at Paris if participates, or from Stockholm. So hell be 100 points up.(and thats optimistic). So Djokovic will need 1900 points. With SF at NY he'd gain 500, if he goes to the final(Nadal's side of the draw) he'd get 750. He had shock exit in Vienna last year, so he'd gain 150 points more, if he wins. SF's in Madrid and Paris, 400 more, if he underachieves 300. He never went to Shanghai, so with SF or final there more 200-300 points.
After we sum them up, he'd get 1600, just 200 points behind Nadal,(optimistic). Or let's say 1300-1400, again just 400-500 points behind Nadal.
With AO knocking on the door next year, and with first two Masters shields on HC's it's not so farfetched idea!