Rabble-rousing bank analyst Mike Mayo has a nit to pick with the board of directors of Citigroup (C - Get Report) -- over CEO Michael Corbat's $15.5 million 2016 pay package.

Mayo, a quarter-century Wall Street veteran, has been jobless since his last employer, the Hong Kong-based brokerage firm CLSA, suddenly closed its U.S. stock-research business in February.

And he's spent some of his free time since then digging through the fine print of Citigroup's annual compensation disclosure ahead of the bank's shareholder meeting Tuesday in New York.

It turns out that Citigroup's board used one group of peer banks to evaluate Corbat's performance and a different group to estimate the average pay of CEOs in the industry.

That may seem like an arcane matter, but the tactic may have had an impact, according to Mayo: helping Corbat to avoid ranking last-in-class on performance, in turn allowing the board to justify a better pay package.

In estimating the median pay for a CEO in the industry, Citigroup looked at a group of 13 large U.S. financial companies - including Wall Street heavyweights JPMorgan Chase (JPM - Get Report) and Goldman Sachs (GS - Get Report) . Pretty straightforward.

But when evaluating Corbat's performance, the board used a different group of nine firms that included floundering European banks Barclays  (BCS) , HSBC (HSBC) and Deutsche Bank (DB - Get Report) .

In 2016, Citigroup's 6.6% return on common equity - a key measure of profitability - ranked last versus U.S. peers. But it came out ahead of the 2.8% return at Barclays as well as HSBC's 0.8% and Deutsche Bank's negative 2.7%.

Deutsche Bank, Germany's biggest lender, had such a bad year that CEO John Cryan got no bonus on top of his 3.8 million-euro ($4.1 million) salary.

Citigroup claims it's not trying to pull the wool over anyone's eyes: "During our most recent stockholder outreach, we learned that investors generally understood and supported our use of different peer groups for these two different purposes," according to the annual disclosure.

But Mayo wouldn't let it go. He attended Citigroup's annual meeting to confront the board - what he sees as a once-a-year opportunity to hold directors accountable.

"If you're going to include lower-performing banks for financial comparisons, why wouldn't you use those lower performers for the purpose of determining compensation?" Mayo said in an interview this week. "At a minimum, it raises a red flag."

At Tuesday's meeting, Citigroup Chairman Michael O'Neill told Mayo that prior to setting its peer groups, the bank checked to see how pay might be affected -- precisely to head off claims that the system was rigged. O'Neill said there was no difference in the median compensation between the two groups. 

"We've done some pretty serious testing," O'Neill said. "There's no advantage from a compensation perspective." 

Whatever the case, Mayo isn't getting paid for his shoe leather; he says he went to Citigroup's annual meeting because he actually enjoys it.

For their part, Wall Street CEOs might be wishing Mayo would go back to work.