We know very little about Schala. Though she is wise, pure and insighful seeming, like a white innate, she still shows some traits of a red innate,such as she makes decisions based on her own feelings and is passionate, she does things on spur of the moment.
Theories:
Schala toned down her personality because she's from Zeal, where everything and everybody is very calm. By Zealian standards, she probably is quite wild and fiery, going to see the earthbounds even though she isn't supposed to etc. She just comes across as being very calm and gentle to Lucca (I say Lucca, since she's the one who mentions it in her letter in CC).
Kid, on the other hand, having the same innately fiery and passionate nature, but raised by another Red innate, Lucca, in a much less subdued environment she ended up with a much more prominent and obviously fire aligned nature.
Theory 2:
When the clone, Kid, was made, she had a different innate. Perhaps because she was born from Schala's passion, she was created as a fiery, passionate girl. Or maybe it was random? Maybe some higher power, or even Schala herself was aware that only a driving, passionate personality would be able to set events in motion.
Theory 3:
Element isn't set at birth, it becomes only set when the person's personality begins to form. Kid became a fire innate like Lucca though emulating her guardian's passionate nature.