UV (280nm) Protein Quantification - Rough estimation of [protein] does it work? (Dec/28/2004 )

I have been using BCA Protein Assay Kit to detemine [protein] in various samples.
I read somewhere that 1mg/ml protein has an abs of ~1.4.

I tried this with 3 different samples all made to 1mg/ml

Chicken Intestinal mucus = 1.770 (close enough)
Lysozyme (Sigma) = 1.775 (so far so good they are agreeable results)
BSA (Piecre Standard and Sigma Powder) = 0.552 vs 0.516 (?!?!?)

Could somebody please tell me the theory on how the abs at 280nm works, what it relies on and why BSA would have a lower abs.

Read this:
http://www.ruf.rice.edu/~bioslabs/methods/...ein/abs280.html