[Nero Germanicus]

DiogenesLamp is correct. Here’s the section of current law on who qualifies as a “Citizen of the United States At Birth,” if born abroad:
“a person born outside the geographical limits of the United States and its outlying possessions of parents one of whom is an alien, and the other a citizen of the United States who, prior to the birth of such person, was physically present in the United States or its outlying possessions for a period or periods totaling not less than five years, at least two of which were after attaining the age of fourteen years”—8 U.S.C. § 1401

[DiogenesLamp]

“a person born outside the geographical limits of the United States and its outlying possessions of parents one of whom is an alien, and the other a citizen of the United States who, prior to the birth of such person, was physically present in the United States or its outlying possessions for a period or periods totaling not less than five years, at least two of which were after attaining the age of fourteen years”—8 U.S.C. § 1401

That might be the current law, but I believe the law at the time Obama was born is more stringent regarding the necessary residency in the US prior to birth of the Child. If I recall properly, it required five years of residency after attaining the age of 14.

Of course this begs for the question, why doesn't a "natural citizen" have a mother residency requirement?

A natural citizen has neither a residency requirement for the mother, or a residency requirement for the child. A natural citizen has to meet no further requirements to keep their citizenship.