In the financial world, surprises are seldom good, as Comerica (NYSE:CMA) shareholders discovered last week. This national -- though largely Midwestern -- bank sprang a little surprise on the Street. In response, investors treated it like a pinata.

While the straightforward earnings number, a decline of about 2%, doesn't look so bad, there's more to the story. The bank saw some meaningful benefits from better credit and a lower tax rate, so "core earnings" were actually disappointing.

For one thing, net interest income was up 4% from last year but down sequentially. Even worse, net interest margin actually fell to 3.8%, when most analysts and investors expected a basically flattish performance. Worryingly, the margin's drop was largely due to lower non-interest-bearing deposit balances -- mostly mortgage escrow accounts, I believe. An ample supply of non-interest deposits has been a big help to Comerica's results in the past.

Non-interest income was also lackluster, climbing just 2% more than last year's level. And while these revenue items were soft, non-interest expense climbed about 20%. Comerica didn't really have a great efficiency ratio before -- not compared with, say, U.S.Bancorp (NYSE:USB) -- but at least other somewhat troubled Midwestern banks like KeyCorp (NYSE:KEY) and National City (NYSE:NCC) seem to be making better progress.

Comerica might manage to outrun or outgrow its problems in markets like Michigan with better growth in California, Texas, and Florida. However, there's plenty of competition in these other markets from big banks like Bankof America (NYSE:BAC) or Washington Mutual (NYSE:WM), and Comerica doesn't have a great track record of same-branch deposit growth or overall expense performance.

I think these shares are cheap for a reason. Operations are still under pressure here, and institutional investors aren't going to be happy getting their fingers burned on that surprisingly poor net interest margin. If you want a turnaround idea, think of something like National City or Fifth Third (NASDAQ:FITB) and let Comerica earn your trust instead.

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Fool contributor Stephen Simpson has no financial interest in any stocks mentioned (that means he's neither long nor short the shares).