Apple's new MacBook line helped the company grow unit shipments in a tough PC market. Source: Apple. 

The Mac computer line that started it all for Apple (NASDAQ:AAPL) is largely forgotten nowadays by analysts. This is ironic given that Mac sales provided Apple's second largest source of revenue last fiscal year, outpacing the iPad which dropped to No. 3. If a recent report from Gartner is true, moreover, Apple's Mac line is also bucking the broader decline in the market for personal computers.

PC sales continue to struggle, but Mac fights the trend
To be clear, it's not a great time for the PC industry. Overall, the presence of capable substitutes (smartphones and tablets) amid an increasingly saturated market have weighed on unit shipment volumes. Overall, Gartner's preliminary results show that the PC market declined to 288.7 million units, an 8% drop from 2014.

The fourth quarter was no better, with an 8.3% drop in worldwide units sold compared to the year-ago period. Out of the six named vendors, only Apple stymied the decline by registering an increase.

Apple shipped 5.67 million units in the last three months of 2015, or Apple's fiscal first quarter. That amounted to a 3% increase over last year's 5.5 million units. This is nothing to brag about, especially from a company that grew its top-line by 28% last fiscal year, but it does show how Apple's high-end value proposition offsets the otherwise secular decline.

Apple's Mac line is not what investors will be watching Jan. 26
Saying that Apple's Mac line is the company's No. 2 business is technically correct -- and the line is growing in importance as the iPad withers -- but statistics don't tell the whole story. This is because rankings don't address scale.

Although Apple's Mac line is the company's second-largest division, it still only contributed 11% to Apple's total revenue in the 2014 fiscal year. Apple's iPhone line, by contrast, accounted for 66%.

More recently, the tune on Wall Street has been decidedly negative in regards to the iPhone's sales growth. In fact, many analysts expect Apple to sell fewer units this fiscal year than last. Morgan Stanley's Katy Huberty expects Apple to ship 6% fewer units. If correct, Apple will need more than 3% growth from its Mac division to stop its top-line from sinking.

But here's the rub: Sagging revenue and earnings seem to be priced into its stock's valuation. Right now, the company trades at a price-to-earnings ratio of nearly half S&P 500. On a bottom-line, relative-value basis, the company appears to be priced for Armageddon, not for a mild decline in its net income which could be offset on a per-share basis thanks to its buyback program.

The fact that Apple outperformed the broader PC market in units-shipment growth is a good sign, and a testament to Cupertino's brand cachet. However, it's not what investors will be looking for come its earnings release on Jan. 26.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.