Topic: Hard anodizing with various Fe % - dull appearance 6063T6

Need help - on the same rack using sulfuric acid hard anodizing Type III - about 2mils - bright buff 6063T6 anodized/sealed parts come out varied shades - we found some 6063T6 parts with 0.10% Fe with bright finishes; parts with 0.20% Fe moderate gloss finish and parts with 0.30% Fe with dull finish all on the same rack same chemistry - same time etc. Is the iron content dispersing the layer of anodize giving different reflectance...anyone have an explanation.