Wells Fargo (WFC), the fourth-largest U.S. bank, reported its third-straight record quarterly profit as lending and deposits increased. The San Francisco-based lender said third-quarter net income for common shareholders rose to $2.6 billion, or 54 cents a share, compared with $1.6 billion, or 49 cents a share a year ago.

Those results outpaced analysts' profit estimates of 37 cents a share, according to Thomson Reuters. And yet Wells Fargo's stock dropped in pre-market trading, possibly because the strong results also hinted at some lingering concerns that many analysts and investors have harbored over its performance.

For instance, Wells Fargo said it added just $1 billion to its buffer against bad loans during the third quarter, less than rivals JPMorgan Chase (JPM) and Bank of America (BAC). That brought its total reserves to $24.5 billion, or 3.07 percent of its loans, also lower than its biggest competitors.

Wells Fargo has seen its nonperforming assets, such as past-due loans that are no longer accruing interest, nearly double from 1.5 percent of its total loans at the end of this year's first quarter to 2.93 percent in the quarter just ended.

These are important metrics of a bank's health, and one knock against Wells Fargo -- even as it has piled up three straight quarters of record profits this year -- is that it's not setting aside enough money to protect itself from future losses if the real estate market's nascent recovery stalls or home prices begin falling again, pushing foreclosures even higher.

It's potentially significant now because Wells Fargo doesn't believe the worst has passed when it comes to losses on loans to beleaguered consumers. That will likely happen sometime in the first six months of next year, the company said in a release accompanying its quarterly results.

But it may take longer, said Wells' risk chief, Mike Loughlin, in the release: "The recovery may take some time to gain momentum and changes in the economic outlook could affect this time horizon."

Chief Financial Officer Howard Atkins said the company's merger with Wachovia, first agreed to just over a year ago at the height of the financial crisis, was proceeding on schedule and "exceeding" expectations. Executives said income from Wachovia businesses, like investment banking, and cost savings from the merger helped boost results in the quarter.