first of all i would like to clarify your confusion. the absorbance or the OD of any solution is taken at a particular wavelength which depends on the color of the solution. protein absorbance at 280 is the absorbance of uv light by aromatic amino acids and all other absorbance at different wavelengths are after some tests like biuret test 540 nm. and 620 nm for albumin test by some kits. here the concentration of protein is dependent on the amount of color developed due to presence of protein or peptide bond. hope this is clear now.