[Mr. Guy]

Originally Posted by Village Idiot

Spoiler:

Reality Marble is a reflection of one's soul, it's current state; rather than a state they are born with.

For example, this is the reason why in HF, Shirou was dying because he was using UBW; while he had Archer's arm + UBW, Shirou became a different person from Archer because he made the choice to protect the one instead of the many, so Shirou's new Reality Marble was being in conflict with Archer's.

Spoiler:

I really misinterpreted that. I had always thought that Shirou's Unlimited Blade Works (granted from the knowledge in Archer's arm) manifested inside of him because he managed to alter his destiny