The market is discounting the stock of premium-priced Thickburger purveyor CKE Restaurants (NYSE:CKR) by 8% this morning over what some will say were fantastic first-quarter results.

Hey, net income did shoot up by 52% to $16 million. Notably, since it pertains to the increase, the company's costs associated with store closings fell from $6.3 million to $0.6 million on a pre-tax basis. Further, sales increased at a moderate 1.5% to $371 million.

Ah, but the real story is not what the company did last quarter. What stinks up the kitchen are recent same-store sales results. CKE posted same-store sales growth, but it fell from 9.8% for the comparable period a year ago to 2.4%. So sales are up, but the rate of increase is way down.

This downward trend was also evident in last year's fourth-quarter results.

So, what about CKE's 2,029 Hardee's stores? For the five sales reporting periods completed this year, same-store sales have fallen from an impressive 10.5% gain for the comparable periods a year ago to a meager 0.1% gain this year.

The falling rate of U.S. same-store sales growth is evident elsewhere in the fast-service burger industry's quarterly results. McDonald's (NYSE:MCD) sales rose by 4.9%, compared with a 12.6% boost last year, while Wendy's (NYSE:WEN) saw sales fall 2.2% after rising by 9.1% last year.

Sonic (NASDAQ:SONC) bucked the trend with its quarterly numbers, released last night. Same-store sales for the latest quarter climbed by 5.5% -- and that was up from a 4.5% gain a year ago. Sonic attributed the results to a strong marketing campaign.

So if Sonic's marketing did so well, why didn't CKE's widely covered advertising campaign boost its sales? That is one reason to be concerned about future sales.

Also worth considering is the premium burger from Jack in the Box (NYSE:JBX). While imitation may be the sincerest form of flattery, competition from other premium burgers may certainly blunt CKE's attempt to have a unique marketing message -- and Jack is a competitor in the backyard of CKE subsidiary Carl's Jr.

Pricing the stock at 15.4 times forward earnings, analysts expect that the company will earn $0.92 a share this year. But given the same-store sales trend, a new case of mad-cow disease, and high prices for gasoline and beef, that earnings estimate might just be too high. The stock's price already is.

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Fool contributor W.D. Crotty owns shares in McDonald's, but he prefers a Whopper when it comes to hamburgers selling for more than 99 cents. Click here to see the Motley Fool's disclosure policy.