Apple's massive success with the iPhone in 2015 is in stark contrast to the dismal sales of the iPad. For this year, iPad unit sales fell 19% and revenues were down 23%. This represents a difference of about $7 billion. What in the world is happening? Below is a statistical analysis of the iPad's poor performance and the possible reasons behind it.

Apple Insider reported that the first possible culprit for the dip in sales that comes to mind is, naturally, Android. It would be convenient to have individual vendor comparisons but that is not even necessary since Android tablet sales went down as well.

According to Canalys, both Apple and Android tablets sales have posted yearly declines in shipments. In Greater China, the world's largest tablet market, sales were down 34%. This only shows that iPad sales were not being eaten up by Android makers.

The next competitor is Microsoft with their brand new Surface devices. In the Canalys analysis, the Surface Pro was included in the “two-in-one” category instead of tablets. In this category, the Surface Pro enjoyed revenue growth and the whole category grew by a massive 77%. However, Microsoft reported that Surface total revenue amounted to less than $3 billion only. This is despite Surface revenues that grew by 117% in fourth quarter of this year.

It would seem that the real culprit is not from outside Apple but from within. The sales of the iPhone 6 Plus has clearly eaten into iPad territory, the same way iPhones gobbled up point and shoot cameras years ago.

In particular, the phablet design of the iPhone 6 Plus gave stiff competition to the iPad Mini, the primary revenue driver of the iPads.A big contributor for this situation is the easier financing options for iPhones by carriers. This is compared to iPads which are typically sold at retail.