When there is a large group (ie the pre chool children) they have maasses more friends, family, school teachers etc. etc. to cast a vote therefore putting them high up in the results table. Whereas someone like Joseph the sole dancer, would not have had such a large pool to vote for him, maybe just family and friends. Same with Rosie. The larger chidrens group of dancers would similarly have had a massive pool of voters.
At one time Simon Cowell said that the boy singer/guitarist was a definite for the final based on his first performance.
Simon Cowell copped out at the semi finals as putting it to the public vote, made it favour the group because of what I have said above. Also the boy, Jack, comedian, while quite funny, was not really comparable with truly talented contestants like the dancer, boy duet, etc. I still want to know the trick to the arrow through the apple as it was definitely a magic trick a la the bullet caught in the mouth trick.