Apple's iPhone 6 (left) and iPhone 6 Plus (right). SOURCE: FLICKR USER KĀRLIS DAMBRĀNS.

It's almost hard to believe, but Apple (NASDAQ:AAPL) is now facing questions related to its iconic iPhone line after its last earnings report. Although the company reported unit sales up 35% on a year-over-year basis, and revenue up 59%, many were disappointed with its 47.5 million shipments, as analysts expected Cupertino would sell 48.8 million units.

Still, such is the insane world of Wall Street, where companies are punished for not attaining goals set by outsiders. Instead of being happy with Apple's iPhone sales growth, missing analyst expectations by a mere 3%, the company's stock dropped 7% in the after-hours markets.

When compared to the overall market, however, Apple is doing just fine. According to Strategy Analytics, Apple is growing its market share in the smartphone industry. Here's the data company's report on second-quarter smartphone sales:

VendorQ2 '14 ShipmentsQ2 '14 Market ShareQ2 '15 ShipmentsQ2 '15 Market Share
Samsung 95.3 22.30% 89 20.50%
Apple 35.2 8.20% 47.5 10.90%
Huawei 20.6 4.80% 30.6 7.00%
Microsoft 50.3 11.80% 27.8 6.40%
Xiaomi 15.1 3.50% 19.8 4.60%
Others 211.5 49.40% 219.9 50.60%
Total 428 100.00% 434.6 100.00%

Source: Strategy Analytics. Shipment figures in millions.

Apple's growing market share, while Microsoft and Samsung are not
While the actual shipments from Apple were already known, the story here is Apple's market-share growth. As compared to the entire industry that grew sales from 428 million in the second quarter of 2014 to 434.6 million in the current quarter -- good for a growth rate of 1.5% (more on this later) -- Apple grew its market share by growing its units-shipped figure by 35%. It appears that Apple stole market share from two big competitors: high-end competitor Samsung (NASDAQOTH:SSNLF), and Windows Phone purveyor Microsoft (NASDAQ:MSFT).

Recently, both companies have acknowledged struggles with their mobile-phone businesses. Earlier, Microsoft took a huge $7.6 billion writedown in its mobile business, as new CEO Satya Nadella moves away from former CEO Steve Ballmer's vision of being a devices and services company. In Samsung's second-quarter results, the company reported a 37% year-over-year operating profit drop in its mobile division. Both companies witnessed year-on-year market share drops where the other listed vendors grew those figures.

Something all parties should watch
While I feel Apple's sell-off was incredibly irrational, Strategy Analytics' data does point to a larger concern many voiced during Apple's earnings -- slowing growth rates in the overall market. As previously mentioned, the growth rate in Strategy Analytics' data is only 1.5%, a slightly disconcerting rate -- especially when compared to the high-growth rates in mobile phone growth during the past decade or so. The risk to Apple investors, as well as every company within this industry, is that a slowing market will eventually hit the company's bottom line as it is hard to swim against the tide.

Between Samsung, Microsoft, and Apple, the one that is probably the most affected by this slowdown is currently Samsung. The company is an electronics conglomerate, but unlike Microsoft, a large portion of its revenue and profit is directly attributable to its mobile-phone business. Microsoft's devices are such a small part of its revenue haul that any future demand decreases are probably inconsequential for the company. The big hit for Microsoft was this quarter's writedown, and that is now out of the way.

To be fair to the bears, Apple probably has the most to lose if demand for its iconic iPhone slows in conjunction with the overall market. During the past three quarters, more than 60% of its revenue has been derived by this single product. But as Apple's earnings and Strategy Analytics' data shows, Apple has grown units sold by 35% and revenue by 59% on a year-over-year basis, while overall units have increased by just 1.5%.

Apple can continue to grow, even if the market slows, by stealing market share. And that appears to be what Cupertino's doing.