It's all because they are independant draws: There is only 1 arrangement to get the 5 women on one tribe but there are many ways to get a 4-1 split:Brenda+Kelly+Holly+Jane w/ Nay on the other side
Brenda+Kelly+Holly+Nay w/ Jane on the other side
Brenda+Kelly+Jane+Nay w/ Holly on the other side
Brenda+Holly+Jane+Nay w/ Kelly on the other sde
Holly+Jane+Nay+Kelly w/ Brenda on the other side
All those have to count in your 4w splits. The numbers increase when you add the men because B/K/H/J could have either Chase or Benry or Marty or etc...
Now with the 3/2 splits, we easily get to the 252 arrangements.
Forgetting the sit-out, I think what you are doing is like throwing 10 Pennies in the air 100 times and seeing how many times you would get 5 heads and 5 tails but that is not what we have here: We have 5 Nickels and 5 Pennies. When the first Nickel lands on Head then all the other Nickels have to turn out Head while all the Pennies have to turn out tails. That's not the same. But I am not a statistician. Maybe Cahaya could help!