According to this:
http://ethnicelebs.com/cory-booker
Booker took a DNA test for the “Finding Your Roots” show, and the results showed:
47% African
45% European
7% Native American (higher than Elizabeth Warren )
It says that both his parents were black, and mentions only one white great-grandparent. But if he’s 45% European, then he must have more white ancestry than that, though where that comes from is not explained anywhere.
It's quite obvious.
His African-American great-great-great-great-parents were probably the offspring of their white slave masters.
Most African-Americans with roots in slavery are 25-50% European.
Unlike the more recent African immigrants.
It's also why African-Americans are varying shades, and not all the very-dark skin tones of most native African citizens. (It also depends on country of origin... but hardly any African-Americans are purely African).