Without having seen the study, I am going to go out on a limb and assume that the gains were better because they were technically 'on' for an extra month while the decanoate ester was still releasing nandrolone.
Esters should cause no change in gains if the drug is active in the blood for the exact same amount of time on each of the subjects being compared. Esters only affect the release rate, they don't alter the drug's effects. I know people say they prefer Tren A over Enan, and prop is better than test enan, etc, but I think mg for mg, the gains should be the same regardless of ester so long as you have the drug in your system for the same time frame.
There are so many other variables it is not very easy to compare two esters gains wise.