I can imagine two possibilities as to why a girl might travel as a boy.
First, the purser who made out the passenger list made an error in
gender. Second, the family thought she'd be safer if she traveled as a boy.
As for the uncle being called father, if she actually traveled as a
girl, the family may have thought it would be easier for him to pick her
up when she arrived. There was a prohibition against young women
leaving Ellis Island with men who were not close relatives (though I
think an uncle would have qualified anyway). On the other hand, that,
too, could have been a manifest error.
Islip, NY USA