Steal SP deals way more damage than normal dagger attacks (OT1)

My level 40-ish Therion deals 1200-1300 damage per Steal SP hit (it hits twice) while dealing only around 400-500 per hit for normal boosted dagger attacks, all other factors being equal (equipped dagger, enemy weaknesses, broken status, buffs from Lion Dance/Abide, amount of boosts etc.). I haven't tested normal attacks with other weapons in the similar controlled situation yet, but the damage should be in the ballpark area of normal dagger attacks.

Is this normal? Am I missing something?