1. Teaching is hard.
2. Teaching abstract concepts (ie. math) is often harder because not everyone thinks the same way. (This is why rote learning in math was the standard for so long.)
3. Teaching abstract concepts in a gender-biased abstract composition of fields (STEM) in a gender-biased country (USA) is harder to teach equally to both genders.
4. Teaching in that context to kids is hardest.
There we go...now you can forget it was an Obama making the claim and come at me instead.