How is this possible? First, it's important to realize this won't be the case for every child of a U.S. citizen born abroad. Certain requirements must be met.
Under current law, when a child is born outside the U.S. to two married U.S. citizens, that child is also a U.S. citizen at birth. This itself didn't help Marco because his mother was not a U.S. citizen. However, the law also provides that it's enough for only one parent to be a U.S. Citizen if that parent had been physically present in the U.S. or its territories for a period of at least five years at some time in his or her life prior to the birth. Also, at least two of those years had to be after the parent's 14th birthday.
Even when the U.S. Citizen parent doesn't have enough time spent in the U.S. other laws can help. One law allows the parent to count time spent abroad as time in the U.S. if that time was spent serving honorably in the U.S. armed forces or being employed by the U.S. government or by certain international organizations. Yet another rule says that time spent doing those activities counts as time in the U.S. if performed by the parent of the U.S. Citizen parent, as long as the U.S. Citizen parent was unmarried and the two of them were in the same household.
Even when the U.S. Citizen parent was not married to the non-U.S. Citizen parent at the time of birth, there are other rules that might apply. But those are more complex and depend on whether the mother or the father is the U.S. Citizen. There are also possibilities for automatic citizenship that can apply after a child is born but before the age of 18.
In Marco's case, he was almost deported because nobody realized he had been a citizen his whole life which, of course, meant he could not be deported. This is why it's an important to have a case examined from every angle by a competent, licensed attorney.