The second-quarter earnings from Bank of America (NYSE:BAC) and Citigroup (NYSE:C) are in. And one number should be a cause of concern for Citigroup investors.

As I've previously mentioned, one of the things that troubled me about Citigroup was its persistently high provision for credit losses. This number gauges what a bank expects to lose on the loans it has written.

In the first quarter we saw Bank of America expected to lose 40% less on the loans it had written relative to where it stood a year ago. Citigroup's provision fell as well, but importantly, it was down by just 20%. And troublingly, the second quarter revealed that the gap only widened.

The numbers won't budge
Bank of America continued to see a significant decline in its anticipated losses. And while Citigroup also saw a fall, the decline wasn't nearly as significant:

Dollars in billions. Source: Company investor relations.

And it isn't just that Bank of America is expecting to write off fewer loans in a raw dollar amount. Citigroup is also continuing to expect a greater percentage of its loans to be written down:

Source: Company investor relations.

One of the most striking things is the reality that one year ago, Bank of America had a larger total of loans it anticipated writing off, but over the past year it has outpaced this reduction in losses relative to Citigroup by more than $1.5 billion:

Source: Company investor relations.

Why this matters to investors
In many ways, the performance of banks is tied to broader economic and market conditions. Knowing the economy has improved, it should come as no surprise that banks like Bank of America are expecting to write off a fewer number of the loans issued.

But in the instance of Citigroup, while we don't know what every bank has done in the second quarter, we do know that in the first quarter, according the FDIC, all the banks in the U.S. saw their provision for credit losses fall by 54% year over year. And the FDIC said these "lower provisions remain a key to earnings growth." 

Undeniably, there is always a delicate balance of risk and reward when it comes to any investment. The fact that Citigroup trails not only peers like Bank of America by a wide margin in this pivotal metric, but also the broader industry average is something every investor should keep in mind.