That's a good point and when I was reading the article I actually predicted that the "Learn More" button would win because it was the least committal but I never drew the connection between that and volunteer signups.

A few theories:

1) Maybe the feeling of not committing by pressing the button made people more comfortable signing up to be a volunteer, figuring that it probably wouldn't be too much of a commitment

2) Maybe the content on the Learn More page was just so good that the only thing that really mattered was getting more people onto that page and they would be comfortable

3) Perhaps the type of person more likely to click after seeing the picture of the family (Stay at home moms perhaps) was more likely to have time/desire to volunteer.

As I think about it, assuming that one of these ideas is right which is probably not the case, I would guess that it's #3. I don't know the demographics of the people who volunteered but around me it seems like it would be the moms who go to the site, and they would probably be more likely to be skewed toward clicking with the family and non-committal button, while also being more likely to have the time to volunteer.

Disclaimer: This is pure, assumption based, uninformed speculation of the worst kind.