Sandwich slinger Potbelly (NASDAQ:PBPB) delivered a surprisingly positive earnings report on Tuesday, bucking the market's seemingly negative prediction during the day's trading.

Since its market debut in October 2013, the company has been one of the more closely followed (and richly valued) quick-service restaurants on the scene. In its earnings release, the company showed investors continued growth and an adjusted bottom line that came in ahead of analyst expectations, with the promise of healthy double-digit earnings growth for the full year.

Still, after-hours trading reacted mutedly, barely reversing Tuesday's 3.5% drop in shares -- what gives? Here are three reasons Potbelly's earnings may not be as good as they look.

1. Same-store softness
Like many businesses in the past few months, Potbelly's management cited extreme weather conditions as a cause for lower same-store sales. Stores open for more than 12 months saw a 2.2% drop in sales, which is understandable considering that the majority of Potbelly's locations are in the Mid-Atlantic and Northeast regions. Still, these aren't encouraging numbers for a company that is trading at more than 35 times forward earnings.

In the company's full-year guidance, same-store sales are only projected to grow in the low single digits. Compare this to the industry champion, Chipotle Mexican Grill. Now, keep in mind that Chipotle is a much larger business that has been around longer than Potbelly and has a tremendous store footprint: 1,600-plus restaurants compared to the latter's roughly 300. In Chipotle's most recent quarter, the company saw same-store sales grow more than 12%. While weather undoubtedly played a negative role in the first few months of this year, investors should not allow it to be used as an excuse for tepid demand.

Management claims the company will hit its target of 25% ROIC, which implies that these stores have much, much growing to do.

2. Margin crunch?
Another factor that investors need to keep an eye on is declining shop-level profitability. In the most recent quarter, the margin slipped by 1.2% to 17.5%.

The company also took a sizable one-time expense (removed from the adjusted earnings) in the form of impairments and closures, costing nearly $900,000. Potbelly defines these as the impairment (i.e., writedowns) of long-lived assets, gain or loss on disposal of property, and store closure expenses. The company did not close any stores during the quarter.

Labor expenses remained steady, while occupancy costs rose nearly 1%. SG&A costs came down more than 1% to 10.6% of sales, implying that the company's marketing efficiency is improving.

3. Too high
Perhaps the biggest reason for the market's response thus far to Potbelly's earnings is that this is a company that traded at unreasonable metrics from the day it went public. Potbelly's stock has shed nearly half of its value during the first eight months of its public run, yet it still trades at a phenomenally high valuation. Growth lovers will note the expected 10% annualized new unit growth, 35%-45% 2014 expected earnings growth, and the aforementioned 25% annualized ROIC. There's no doubt that the company is growing, and at an impressive clip, but the market may have priced in all of that growth and then some.

Investors need to keep in mind the value received for the price paid. Potbelly, as a business, has a bright future. The market has yet to embrace it, though, as the required growth to justify the multiples is astronomical.

Michael Lewis has no position in any stocks mentioned. The Motley Fool recommends and owns shares of Chipotle Mexican Grill. 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.