Given that Solomon Northup’s father was born a slave, how is Solomon Northup a free man?

Solomon was born a free man in a free state. His father, a slave, was freed upon the death of his master and moved to New York, where Solomon was born.

Having been born a freeman, and for more than thirty years enjoyed the blessings of liberty in a free State—and having at the end of that time been kidnapped and sold into Slavery, where I remained, until happily rescued in the month of January, 1853, after a bondage of twelve years—it has been suggested that an account of my life and fortunes would not be uninteresting to the public.

Sometime after my father's liberation, he removed to the town of Minerva, Essex county, N. Y., where I was born, in the month of July, 1808.