Well you can look at it the scientific way. In gender genes boys are XY and girls are YY. Boys have more differenting genes which means it's easier for many learning disabilities and conditions to affect them.
you can look at it socially. At a younger age, girls have higher academic expectations, since they mature faster. Its expected that they're organized while boys can often get teased by this. Since girls grow up with this and develope these skills, they cope better with ADD or ADHD.