Soc Gen analyst says there's 'trouble brewing' as Apple iPhone 5S sales made a 'puzzling' jump this quarter

Apple's iPhone 5S is selling much better than expected, cutting into sales of the iPhone 6 and 6 Plus, according to Societe Generale analyst Andy Perkins.

"There appears to have been a pick-up in demand for the older iPhone 5s model. Sales of this model increased from 10% of total units in Q1 FY15 to around 20% by Q2. This is unusual," says Perkins in a new report. "Normally demand for older models never recovers once the new model is released ... this observation remains slightly puzzling."

Perkins' data comes from monitoring handset exports of Hon Hai, and Pegatron, two handset manufacturers contracted by Apple.

If Perkins is right, this could be a problem for Apple since the 5S is cheaper is a $100 cheaper than the iPhone 6, and $200 cheaper than the 6 Plus.

In addition to increased 5S sales, Perkins notes that the dollar is getting stronger. As the dollar gets stronger, it's going to hit the average selling price (ASP) of the iPhone. Perkins thinks the ASP of the iPhone is going to drop to $651 from $687 in the previous quarter.

Of course, this happens every year. In the holiday quarter, iPhone ASPs are strong, then they drop in the March quarter. Even if Perkins is right, and the ASP drops to $651, it's still going to be one of Apple's best selling prices in years.

Beyond concerns about the March quarter, Perkins thinks "trouble is brewing" because he doesn't believe Apple will be able to grow iPhone sales after last year's blockbuster launch. The iPhone 6 had a bigger screen than the iPhone 5S, which led to a massive jump in sales. Apple doesn't have another easy way to upgrade the phone to goose sales.

"The most obvious hole in the previous iPhone portfolios (a variety of form factors) has been filled," says Perkins, adding that geographical expansion will not make much of a difference since Apple is already selling the iPhone through almost all major mobile providers.

But what about the Apple Watch? Can't it make up for any potential softness with the iPhone? Nope, says Perkins: "We do not believe that the Apple Watch will be a major game changer for Apple, at least over the next 12 months."