400 years of slavery?

If we generiously count 1775 as the beginning of the US, slave importation was banned in 1807, Slavery was finaly banned in the Union slave holding states by 1866 (the last states where it was legal) How in the hell does this equal 200 or the oft quoted 400 years of slavery in the US? Prior to that the British, French, Dutch or Spanish govts were responsible for it.