When people say "we did our best," they mean they worked really hard to accomplish something, right? So by going through the strenuous process of mating with each other, through their hard work Zuikaku and Shoukaku made a lot of Shou-cuckoos and Zui-cuckoos. That is their accomplishment.
The picture above that one seems to explain it well.
Thank-you-for-Why Kaga-san?!Nnsho*bluuush**fluffy*Pya!Just for the winter...
She gets attached to me.ღ Nice and toasty ღHnngh...*fluffy*warming-me-up bean sprouts.NnshoWe did our best.Zuikaku...