Djokovic had too many ups and downs during 2012 - 14 period. Otoh Federer only lost to a zoning player or Nadal during 2004-07. So I can see Djokovic dropping many slams no matter of opposition (like W/USO 13 - 14, Uso 12). He takes 6-8 slams vs Fed's 11 in four years.
This analogy is improper because you cherry pick Djokovic's relatively 3 poorest years out of 5 being considered but you don't do the same for Federer and then act as if it's a fair comparison. The OP is looking at 2004-2008. If you cherry pick Djokovic's 3 poorest straight prime years, 2012-2014, you have to do the same with Federer, 2006-2008. So let's look at that:
Federer's overall win percentage in 2006-2008 is 88.4%.
Djokovic's overall win percentage in 2012-2014 is 88.1%.
That's not a significant difference, but Djokovic faced much tougher competition in those years. 2006-2008 Federer played someone in the top ten 61 times. 2012-2014 Djokovic played someone in the top ten 88 times. That's 27 more top ten matches.
Federer's win percentage vs. top ten in 2006-2008 is 70.5%.
Djokovic's win percentage vs. top ten in 2012-2014 is a notably better 76.1%.
Now let's see who they lost to outside of the top 10.
2006-2008 Federer lost to:
11 Andy Murray
21 David Nalbandian
21 Andy Murray
22 Ivo Karlovic
22 Gilles Simon
25 David Nalbandian
27 Radek Stepanek
53 Filippo Volandri
55 Guillermo Canas
60 Guillermo Canas
98 Mardy Fish
2011-2012 Djokovic lost to:
11 Kei Nishikori
11 John Isner
15 Jo Wilfried Tsonga
18 Tommy Haas
20 Tommy Robredo
22 John Isner
23 Sam Querrey
28 Grigor Dimitrov
So 2006-2008 Federer lost to more players outside the top 10 than 2012-2014 Djokovic and lost to a lower caliber of player. 2012-2014 Djokovic never lost to any player outside of the top 30 in all 3 years. Federer lost to Volandri, Canas twice, and Mardy Fish, all outside the top 50 at the time.
So in a proper analysis, 2012-2014 Djokovic is actually more consistent than 2006-2008 Federer.
Now let's do the analysis for all 5 years. 2004-2008 Federer had an overall win percentage of 90.6%. 2011-2015 Djokovic had an overall win percentage of 89.8%, less than a 1% difference. But against the top ten, Federer went 76-20 or 79.2%. Against the top ten, Djokovic went 119-30 or 79.9%. But ridiculously, Djokovic played the top ten 149 times to Federer's 96, so 53 more times! This is when Djokovic had much tougher competition than Federer too as Murray, Wawrinka, Del Potro, Berdych, Tsonga, and Ferrer were all prime or peaking. Add on Nadal and Federer himself, and 2011-2015's level of difficulty is much greater than 2004-2008's.