wolf
i changed my mind
i think its a good kasha
and i believe i have the answer
you know the Gemorrah re a granary of produce. you can ask for the amount to be larger than it actually is (changing the past) but NOT if you already measured it and know how much is there.
so Hashem DOES change the past in certain circumstances but CHOOSES not to do so in other circumstances, apparently when it involves rewriting our daas.