Equilar noted a primary driver of realizable pay besting reported pay is a positive total shareholder return (TSR) over the last three years. For example, McKesson's TSR, which includes stock price appreciation and dividends, was 36.8 percent for the three years ending in December, 2012.
A positive TSR though is not always a guarantee that realizable pay will exceed reported pay.
(Read More: Slideshow - 2012 Highest Paid CEOs)
Take Larry Ellison's realizable pay for the last three years. By Equilar's calculation his realizable pay of $109 million dollars is 41 percent of his reported pay of $243.9 million dollars, despite the software giant's total shareholder return being 11.5 percent for the three years. Equilar said the difference between Ellison's reported and realized pay can be attributed to Oracle valuing Ellison's options at far higher levels than they were at the end of 2012.
Realizable pay is likely to be a hot topic this proxy season. With executive compensation still a focus of shareholders, the proxy advisory firm ISS will be looking at the realizable pay of S&P 500 CEOs, who's reported pay raises red flags in ISS's primary analysis. Glass Lewis, another proxy advisor, also plans to use realizable pay when it analyzes CEO pay, feeling it sheds much needed light on what an executive will ultimately earn.
—By CNBC's Mary Thompson; Follow her on Twitter: