no idea. could be a the protein boost. Red meat does have the nickname "natures multivitamin"
Red meat contains a lot of iron. And while iron also comes from vegetable sources, red meat contains more iron than most foods and is best abrsorbed by the body in this form.
Red meat also contains B vitamins, phosphorus, potassium, magnesium, selenium -- and protein, critical for muscle and organ health. Protein from red meat is complete, meaning it contains all the amino acids the body cannot make on its own. Protein helps the body repair and renew.
I've hear of selinium being put in chicken feed, maybe that's it? Besides the fact feeding them meat gives them plenty of extra goodness to put through to their eggs.