Unless there's some sort of difference in the way people are affected when they're attacked. We've already seen that there are different sorts of zombies, who is to say that all victims of that one zombie will have the same reaction? We've already seen that not all people are affected in the same manner.
We know that it is a second generation Little One that is responsible for causing all the mayhem and carnage at Boulder. What if the zombie strain/infection/bacteria/whatever-other-cause can devolve in the same way we've seen it evolve? Perhaps a third, fourth, twenty-second, etc generation Little One attacked someone but they just weren't AS potent as an Inkling.
Going back to some of the earlier comments about how there should not have been any possible way for Datu to have taken down a Little One, this might explain it. Perhaps as the virus/zombie-ism/etc is passed on from one person to the next, it becomes diluted. If this is the case, the one Datu killed could have been a lesser version of an Inkling. Not quite as strong or fast, with thinner skin than #2.
Just tossing out some ideas.