It's more inner workings than just item levels. Indeed, the item level on crafter and gatherer gear is usually shown to be lower than their disciple of war/magic counterparts, but they count as their equal. For example, wyvernskin is largely i125 equipment, but a wyvernskin jerkin (gathering chest) is classified as i80. But that still counts as i125 when desynthesizing because it's in the same category of gear as the other i125, that of wyvernskin. Similarly, some glamour equipment that require high level crafting, even though they are shwon as item level 1, are indeed considered high level internally. For example, the "Expeditioner" gear set (minus the cap) counts as i210 because it's a three star level 60 craft, but it's shown as item level 1 like all glamour gear. You probably want to look at this set to level your LTW desynthesis in that level range.