There is good reason for this. Justice Gray [you can tell from his writing] desperately wanted to declare Ark NBC. To that end, he bastardized, exaggerated, and even lied about the meaning of some of the citations in the opinion [Calvin's Case, Dicey, Blackstone, etc.] So, why didn't he do it [declare Ark NBC]?
There is only one possible explanation - he WOULD NOT have gotten a majority on the Court if he had insisted that Ark was NBC. They were willing to go along with Ark being a citizen under the 14th Amendment [which, he clearly was] - but NOT NBC ...
In lawyer-speak, they call it "splitting the baby". Ark got half a loaf - but it was better than none ...
And in the process set a SCOTUS precedent that someone born to non-citizen parents is not “natural-born.”