Soul Trapping Scroll vs Spell

[Lightning Rod]

I haven't been playing very long, but I noticed my Soultrap Scrolls always work, even on Golden Saints, but my Soultrap spell is often resisted or reflected. Is this because of my spell casting abilities? I thought my ability affected just casting the spell, success or failure, not it's power. If you were able to cast the spell it had a set power based on the "roll" not on your ability. Does this apply to all spells?