Samsung (PINK:SSNLF) just reported quarterly earnings that look pretty spectacular from 10,000 feet … but on closer inspection show some worrisome signs.

Year-over-year, the 7.15 million won (about $6.4 billion) net profit for this year’s first quarter was an improvement of 42%, which certainly is impressive. However, that’s a far cry from the 81% improvement from Q1 2011 to Q1 2012. It’s also just a 1.4% improvement from Q4 2012’s 7.05 trillion won.

What all of this means is that Samsung’s pace of growth has been slowing — a troubling trend for shareholders, who have enjoyed 60% gains over the past two years but have seen the stock flatten over the past few months.

And while smartphone sales tend to slow in this quarter as consumers anticipate new releases, Samsung’s reliance on cell phone profit actually increased slightly — from 73% to 74% — suggesting it’s leaning even more heavily on mobile. (Bad news, considering Samsung has warned investors about increased mobile competition — a threat to the segment that dominates its profits.)

Samsung has been going gangbusters on selling phones, but the company makes dozens of different models, many of which are cheap feature phones — good for overall market share, which is how the company unseated Nokia (NYSE:NOK) after 12 years to become the world’s top cell phone vendor — but not so good for profits.

The real money is focused around its premium smartphones, and the upcoming Galaxy S4 is facing increased competition, not just from Apple, but from other Android smartphone makers. Companies like HTC, Sony (NYSE:SNE) and Google (NASDAQ:GOOG) are taking a run at the premium market this year.

Given Samsung’s heavy reliance on these devices for profits, any gains by those vendors will have a material effect on Samsung’s bottom line.

Apple (NASDAQ:AAPL) has this problem too, with the iPhone estimated to account for two-thirds of its profits. However, a key difference is that Apple is a much more focused company in terms of product lines. Outside of the iPhone, it has the iPad (where sales continue to surge, growing 65% year-over-year in the last quarter), iPods, Mac computers and iTunes. That’s it.

Samsung has dozens of different product lines, including tablets, PCs, computer monitors, digital cameras, camcorders, home appliances, TVs, stereo equipment, printers, memory cards, hard drives and LED lights — all while operating its own digital media storefront. It also manufactures many of the components that go into high-tech products made by other companies, such as CPUs, memory chips and LCD displays.

Despite that diversification, Samsung’s revenue for the quarter totaled 52.87 trillion won (roughly $47.3 billion), which is not much more than the $43.6 billion in revenue Apple reported.

Another way of looking at this is to consider the impact if smartphone sales were to drop significantly. Apple would take a hit in this scenario, but its iPad sales are robust, its other divisions make money and it has room to expand its product line into something else (like TVs) if needed.

Even with the iPad’s lower margins — estimated to be in the 37% to 40% range — Apple would still be a highly profitable company if smartphone sales fell. Investors wouldn’t be happy, but the company wouldn’t be in trouble.

Samsung’s plan B is hazier. While it dominates mobile phone sales with nearly 33% of the smartphone market, Samsung is a relatively small player when it comes to tablets with a roughly 8% share. Even though it hopes to double last year’s number (to roughly 40 million tablets for 2013), Apple moved half that many in Q1 2013 alone.

Sure, Samsung has all those other product lines, but even the ones where it is a dominant player — TVs, for example — it’s not performing well. TV profit margins were 2% in the last quarter; better than losing money, but nothing to brag about.

There are no real new product silos to pursue; Samsung has tried it all, and it’s not making much from those other lines of business. This huge range of established products can be looked at as an advantage, but it also could become a drag on earnings if those smartphone dollars disappeared.

One interesting point of comparison between Apple and Samsung is the average profit margin they have on their mobile devices. Apple has been able to maintain margins in the 49% to 59% range for its iPhones, while the Wall Street Journal says Samsung hit 19.8% in Q1.

That’s how Apple can snap up more than 70% of the smartphone market’s profits despite Samsung being the world’s largest mobile phone manufacturer with sales nearly double the number of Apple’s units.

What makes this a bit of a head scratcher is that Apple and Samsung sell their smartphones for about the same price, yet Apple is buying key components like the A6 processor, system memory and flash memory from Samsung. Presumably, Samsung is making a profit from Apple on these parts and would be able to supply them for its own devices at cost. So how Apple is managing such a lead over Samsung either speaks to CEO Tim Cook’s reputation as a supply chain genius, or suggests startling inefficiencies at Samsung.

Investors obviously aren’t worried about Samsung’s reliance on smartphones as a profit center. While Apple stock is down 40% since last fall, Samsung’s (which you can’t buy directly on U.S. markets) is up 8% over the same time period.

The company could conceivable wring more efficiencies out of its operations to boost margins, but given the converging trends of increased smartphone competition and Samsung’s increased reliance on this mobile segment for making money, it seems entirely plausible that Samsung stock could continue to come up flat — if not worse.

As of this writing, Brad Moon did not hold a position in any of the aforementioned securities.