I've never seen it used to refer to a bunch of girls, but here's a thought:
Could it be a remnant of when we all spoke latin?
In spanish, singular pronoun to describe a male is el
Singular pronoun to describe a female is ella
Group of males is ellos, and group of females is ellas.
However, you could have a group of 10,000 women, but if there is even one baby boy, grammar dictates you use ellos as if it were a whole group of men.