I just bought an HQ Longarm's Composite Bow (Level 50), and I'm trying to put a Savage Might Materia III on it. But when I try it shows Determination +0 when it should be +4.

My understanding is that to meld a tier 3 materia, the item needs to be item level 45. But the bow I just bought it is item level 55 so either this makes no sense, or I'm wrong about one of these numbers (probably the latter obviously).

So if someone could kindly explain what I'm wrong about it would be greatly appreciated.