They also never set their goal as high. Seleucus settles for what he has, he does not attempt to build an as large an Empire as the one Antigonus had but rather divides the spoils and is content to consolidate his gains.
Seleucus and Antigonus are similar in some ways, neither of them were very likely to be seen as serious contenders at the start of the conflict of the successors, and both more or less created their empires out of their own abilities.
Both were also incredibly lucky at times. For instance Seleucus reclamation of Babylon with just a few trusted men was not in any way something that had to work out like it did.
The Antigonid success on the propaganda front should also not be discounted. Where all successors claimed to uphold the freedom of the Greek cities Antigonus seems to have been the only one to actually be able to be seen as a credible champion (despite Demetrius conduct in Athens), something both Cassander and Ptolemy failed spectacularly at.
At the end of the day it will be hard to compare the qualities of people like these to each other, I am not sure realistically it could be done. The diadochi were all exceptional people, if they had not been they would not have lasted long. The fates of the many other involved people during these wars clearly show that you were always just one mistake away from losing everything and being killed, no matter how shrewd your plan or how successful you were up to that point. This last part is something I would keep in mind about Demetrius as well, he had ample opportunity to fail but all in all did pretty well for himself, much unlike the sons of Cassander for instance. Failing against veteran generals of both the successor wars and Alexander's wars before that, is not necessarily a sign of lacking competence
Now, since I actually know a thing or two about why the stats currently are what they are I will add that any stats you see are unlikely to be the final product, both because they might be tweaked and because the traits they have may be tweaked themselves