[nehalita]

Just a thought:
After reading these comments, many of which I agree with, I'm making a hypothesis.

There is confidence in numbers.
Boys see other boys doing mechanical work so they feel more invited to do the same.
If there are girls also doing mechanical work, the next girl might feel encouraged, but if there are only boys working on the robot, then the girl would have to break the "invisible barrier". It may be, in fact, that there is no barrier, but to a girl who sees only boys working or designing the robot, they might have a preconditioned barrier of their own to cross.

This could be with anything. What if only girls did the electrical work and always do? I'm sure some boys might hesitate and do something else instead.

Just throwing this out there....