For Apple (AAPL -0.60%) investors, perhaps the best omen for future success is the company's first weekend sales figures for its signature iPhone units. And now the numbers are out, it seems Apple met, and slightly exceeded, high expectations by reporting 13 million units sold in the first weekend of availability for the newest-gen iPhone 6s and iPhone 6s Plus units.

On a year-over-year basis, however, Apple investors appear to have something to celebrate. When compared to last year's figure, 10 million units, this year's 13 million figure shows solid growth from a refreshed unit. The comparison is even better when growth is factored in: Last year's units-sold figure of 10 million was an 11% increase from 2013's 9 million unit figure whereas this year had a 30% jump.

And while Apple investors should be happy the company sold 13 million units, the 30% growth figure is a slightly unfair comparison.

It would be a great comparison, if it was an equal one
When using comparisons, it's important to make fair and just ones in order to determine performance. And for Apple, there's a notable omission when it comes to initial iPhone availability, as detailed by the press releases over the last three launches below [bolded print and italics added for emphasis]:

Both iPhone 5s and iPhone 5c are available in the US, Australia, Canada, China, France, Germany, Hong Kong, Japan, Puerto Rico, Singapore and the UK

iPhone 6 and iPhone 6 Plus are available in the US, Australia, Canada, France, Germany, Hong Kong, Japan, Puerto Rico, Singapore and the UK

Both models [iPhone 6s and iPhone 6s Plus] will be available in Australia, Canada, China, France, Germany, Hong Kong, Japan, New Zealand, Puerto Rico, Singapore, the UK and the US beginning Friday.

So as you can see, Apple introduced the newer iPhone in two more geographies this year. Setting aside the economic powerhouse of New Zealand for now (sarcasm intended), the bolded country of China is the biggest reason Apple's headline number should be discounted. Simply put, comparing this year's numbers to last year's, when the iPhone 6 and iPhone 6 Plus were delayed in mainland China due to licensing issues, is a materially unfair comparison of the first week's sales numbers.  

For an apples to apples comparison (pun shamelessly intended), FBR Capital Markets analyst Daniel Ives pegs Apple's China sales at 2 million-2.5 million units, making an ex-China figure at the midpoint 10.75 million, a smaller 7.5% increase than last year's 11%, if Ives estimate is correct.

As far as investors are concerned, it seems Apple is continuing its success in the Middle Kingdom. For a comparison of Apple's immense growth in China as compared to the prior year, see the aforementioned chart:

Source: Apple's Quarterly Report . Apple's China revenues includes Hong Kong. Revenue figures in millions.

More time equals more sales
But there's also a second -- albeit it less important -- reason why comparing this weekend's sales to last year's is (slightly) flawed as well. And that has to do with the length of the preorder period. This year preorders started on Sept. 12, giving potential customers nearly two weeks to order the newest model, which started shipping on Sept. 25.

Last year, preorders started on Sept. 12 as well, but the device went on sale on Sept. 19, only giving shoppers a week to place the initial order. While this isn't as important as China's inclusion, the longer preorder period as compared to last year's should have led to increased incremental sales.

As an Apple investor, I'm happy with the 13 million figure, but I'm holding off on calling the first weekend sales "phenomenal" like CEO Tim Cook stated. In the end, this is a strong result for Apple, but probably not as good as the headline number suggests.