Why does sequence of amino acids presented on the Catalytic Site Atlas from a given protein differs from the sequence on the RSCB Protein Data Bank
Crystallography results (pdb files) almost always contain a truncated sequence. 

Both ends of a protein are often flexible (even in a crystal) and don't result in enough data for a good fit. The corresponding residues are removed from the model and the sequence, and you're left with only the residues that show a defined electron density.