For one thing, as ASpring mentioned, the text about white kids were shortened, with the implication that the same sentence structure as for the black kids should be used. I'm assuming it was done to make the drawing easier to read (i.e. less text) but it certainly added confusion.
I.e. it reads "25% of black kids had parents who grew up in the bottom two levels and moved up at least one" and it should read "59% of white kids had parents who grew up in the bottom two levels and moved up at least one" instead of just "59% of white kids moved up".
As for "Those two groups should sum to less than 100%, certainly not 103%.", this is incorrect: the 25% refers to the kids who had parents in the bottom two levels, while the 78% refers to kids who had parents in the top three levels. Those are separate groups. (same for the number with white kids)
So, if you look at kids coming from the 40% poorest (unclear if it's overall or amongst white/black people), from 1955 to 1970, only 25% of the black kids moved up while 59% of the white kids moved up.
And if you look at kids coming from the 60% wealthiest, 78% of black kids moved down while 43% of white kids moved down.
In other words, poor black kids are more likely to grow up and stay poor than poor white kids. Rich black kids are more likely to grow up and become poorer than their parents than rich white kids.
To simplify a bit (not 100% accurate with the available data but close): regardless of poverty level when growing up, only 22 to 25% of black kids moved up (or stayed the same), while 57 to 59% of white kids did.