One explanation distinguishes faith's foundation from its accompanying light, tranquility, reverence, stability, and obedience. Acts of devotion may enhance these qualities, whereas wrongdoing may impair their expression.
Example: Two people assent decisively to the same truth, but one displays greater reverence and more consistent obedience. The difference in conduct or spiritual condition is real; a separate argument would be needed to show a numerical difference in the underlying assent.
The phrase “faith increases” must be interpreted in light of context rather than forced into one technical meaning everywhere.