I guess the point about Slams in particular is that he has to string seven performances together, including typically at least one stellar performance against another of the Big Four (and, with the form Djokovic and Murray are in against the rest of the field, probably two in succession). That's a tough ask, and indeed Djokovic probably got a bit lucky with the draw at Wimbledon as he only ever had to play one out of Murray and Federer (and Nadal, if he were ever to be competitive at Wimbledon again) this year. Still had to get past Anderson etc, but with the "Big Four" currently shaken up a bit, then draws can make a potentially large difference. On the other side of the coin for Djokovic, at the French Open he had to face Nadal and Murray in succession -- while he successfully got past both in the end, it's possible that those games took it out of him mentally -- enough for Wawrinka to be able to take advantage and win the title outright. Maybe there, the draw hurt Djokovic a bit.
Not that this means Djokovic's win isn't impressive, of course. Like Murray in 2013 he had to come back from two sets down once, and then there were moments in the Final that could have led to huge swings, so he did a great job to win it again.