U.S. Bancorp (NYSE:USB) has released its first-quarter earnings, and overall things look pretty good for the large regional bank. It earned $1.43 billion, or $0.73 per share, right in line with analysts' expectations. Nevertheless, shares have been down as much as 2.6% this morning, prompting a deeper look into that earnings release. Here are three reasons investors may not be that pleased with the big Minnesota-based bank.

1. Net revenue
While net income increased by 6.7% from the same quarter last year, the same cannot be said about the top line. For the quarter, U.S. Bancorp recorded total net revenue of $4.9 billion. This was a 4.7% decline from the fourth quarter and a 1.1% decline from the same quarter last year. 

Noninterest income, which we'll cover in depth shortly, was primarily to blame. It was down 7% from the fourth quarter, and 3.3% from the same quarter last year, primarily because of lower mortgage banking revenues, as well as seasonally lower payments-related revenue.

2. Noninterest income
As mentioned previously, the largest reason for the decline in noninterest income was lower mortgage banking revenues. These revenues declined to $401 million during the quarter, down from $476 million during the previous quarter, equating to a 15.8% decline. This was in spite of $57.3 billion in new lending activity, including $27.9 billion of mortgage and other retail loan origination, proving that new loan activity does not automatically equal high revenues.

3. Net interest margin
Similar to JPMorgan Chase, U.S. Bancorp experienced a decline in net interest margin. At 3.48%, the bank's net interest margin fell by seven basis points from the fourth quarter, and by 12 basis points compared to the same quarter last year. With U.S. Bancorp reliant on interest income for over 55%of its total revenue, the decline in noninterest income was not solely responsible for the bank's decline in revenue this quarter. However, the longer that the Federal Reserve's quantitative easing continues, the bank may need to beef up operations elsewhere to mitigate some of these losses until its net interest margin stabilizes.

The Foolish bottom line
Meeting expectations often is not enough to satisfy the Street when it comes to earnings. That's why it is important for investors to look beyond the first paragraph of an earnings release and try to get a grasp on what is happening "behind the scenes" at their favorite companies. In the long run, U.S. Bancorp will probably be OK despite today's earnings blip, and it could be a great addition to any portfolio.

Fool contributor Robert Eberhard has no position in any stocks mentioned. The Motley Fool owns shares of JPMorgan Chase. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.