Could it be because of the strength that a soul gets after one of those battles?

[ he hums, thoughts running wildly like number ciphers on a short-wave radio frequency. ]

I don't know if there's any way to quantify the strength of a soul nor do I think that we can compare it to the pre-Oracle-slash-timeskip soul, unless you somehow took measurements while we were asleep—which would be weird, not gonna lie—but it kind of feels like how exercising makes your muscles stronger and builds resistance. Maybe the same thing can happen to our souls when faced with challenges?