Answer:
Because you are recounting people several times. Say for example, let us number those people
{1,2,3,4} for the four girls and {1,2…7} for the boys.
One possible combination by your way is to fix one boy and girl each, say
{1,1} and then choose the remaining people. Let this combination be, for the sake of the example, {1,1,2,2,3}. This is a valid combination.
Now, when you fix another combination of a boy and a girl, say {1,2}, you could get the same combination as before because one the combinations while choosing the three remaining people would be {1,2,3}.
A correct way would be to choose girls and then choose 5− boys for 1≤≤4
So we get ∑4=1(4)(75−)=441
