Question About Cures Being Inconsistent.

Just a random question, i'm not a whm by any means, however I decided to put a little more work into my macros. I decided to head off to abyssea and do some testing. When I cured my alternate time after time(without any gear changing) why do the numbers alter? On Darksday I was hitting 1366 with a C6 however i'd have these random spikes of 1229. What's happening?

Cures are affected by day/weather, so darksday was being evil :(

Thank you very much, I should have known that ><
Much appreciated!