There are hundreds of threads about eligibility.
The historical understanding of NBC has always been:
Two citizen parents
Born on US soil
That is, until the usurper appeared on the scene.
That was my understanding also. But, the issue of being born on a US base overseas or being born to two US parents while traveling, etc. has come up. Then people were saying NBC is anyone who did not have to become naturalized.
I always thought NBC meant born in the USA with two parents that were already US citizens.