I think the 5.75% return is over the 48 month vesting duration, on the extra money you would have earned (and invested). So, the excess (per year) would be something more like (50(1.0575^3)+50(1.0575^2)+50*(1.0575^1))/4. (Though it seems I'm a little off, so maybe they do the calculation monthly or biweekly).

Edit: Search http://www.tejusparikh.com/projects/equity_calculator/js/app... for bigCoInterest.