I haven't researched this but I'll heres something and this info is from forums not from code:
Basescore is counted from partial scores (If i rememb multipliers right)
(your pop/max pop without any improvements)*4000
(your land/max land)*2000
(your techs/max techs without future techs)*2000k
(your wonders/max wonders)*1000
Where each pop point is 1, land square is 1, wonder 5pts and anc tech 1,classical 2,...,future 7
Then firaxis score= Difficulty modifier*basescore^current turn/total number of turns.
When they added more turns you can have some victory conditions earlier as you need only same amount of moving to make conquest theoretically therefore increasing score.
I don't know if they changed scoring anyway in BtS but at least it makes score vary alot as there are more turns, more techs and more wonders.
-Dracandross