Top electronics chain Best Buy disappointed investors by not raising its full-year forecast despite an 8.2 percent rise in December same-store sales, signaling a potential weakness in margins.

The muted outlook came a day after better-than-expected December sales results at many retailers and sent Best Buy's shares down more than 3 percent to $40.12 on Friday.

Best Buy's margins have been under the microscope as investors doubt the effectiveness of its aggressive promotional strategy to grab market share and worry about future avenues of growth.

At first blush, you would think the strength in the numbers would lead to an increase in outlook, said Standard & Poor's analyst Michael Souers said, who has a hold rating on the stock.

Strong demand for notebooks and mobile phones helped Best Buy's domestic and international same-store sales rise 9.3 percent and 3.5 percent, respectively. It still chose to only maintain its prior forecast that called for a fiscal 2010 profit of $3.00 to $3.15 a share excluding items.

The two fastest-growing segments for the company are also the lowest margin, FBR Capital Markets analyst Stephen Chick said in a note.

Though the domestic same-store sales was in line with his estimate of a rise of 8 to 10 percent, they ought to have been better considering that gift card purchases were up, said Chick, who has a market perform rating on the stock.

FTN Equity Capital Markets analyst Anthony Chukumba, who has a buy rating on the stock, called the results strong but said investors were likely hoping for better.

Yesterday you had all kinds of different retailers report better-than-expected December sales results and increase their guidance. I think people might have been expecting the same from Best Buy and may be a little bit disappointed that that did not turn out to be the case, he said.

On Thursday, U.S. retailers tracked by Thomson Reuters posted better-than-expected December sales and many raised profit forecasts as they benefited from a late holiday shopping surge and discounts that were more modest than a year ago.

Best Buy is scheduled to report fourth-quarter results and provide its initial 2011 earnings and new-store opening forecast on March 25.

(Reporting by Dhanya Skariachan, additional reporting by Ben Klayman in Chicago; Editing by Dave Zimmerman, Phil Berlowitz)