It's based on gear level, theoretically the higher the item level of the gear you equipped them with, the greater the chance the retainer will bring back better items (the game even states this upon completing a venture, that 'better gear will yield better items' (paraphrased).

Unfortunately each time the highest possible ilevel for gear is raised with content patches, this then also means that retainer gear that was satisfactory, now isn't. As long as you're not equipping a level 70 retainer with item level 1 gear you should still be getting reasonable items - any green and blue gear they bring back that you have no use or need for just trade in for seals at your GC, and sell everything else. Even if stuff only sells for small amounts it's better than nothing.