Apple CEO Tim Cook. AP Apple's iPad sales for last quarter fell far below expectations. It sold 16.35 million of them, but analysts were expecting about 19 million.

On the surface, 16 million sounds like a lot of iPads, but growth in Apple's iPad business has flatlined. In fact, growth was negative for last quarter.

So, what the heck happened?

On a call with analysts this afternoon, Apple CEO Tim Cook did his best to explain.

For last quarter in particular, he said the company reduced its iPad channel inventory compared to the same quarter last year, so sales were actually in line with the high end of Apple's internal expectations.

Speaking on the iPad business as a whole, Cook made some really interesting points to remain bullish. First, he said the iPad is Apple's fastest-growing product in the company's history. Apple has sold 210 million of them so far, which is almost twice as many iPhones Apple sold in the same period of time.

Cook also made a strong case for the iPad in the enterprise market. He cited one study that said 91% of tablets activated in the enterprise are iPads. Meanwhile, nearly all Fortune 500 companies use iPads. He also said it was a smart move of Microsoft to finally release Office on the iPad, which should help with enterprise adoption since many businesses rely so heavily on the software. In fact, Cook said Microsoft should've released Office for iPad earlier than it did.

In education, Cook said the iPad has a 95% market share, but the challenge now is to get more schools to buy them and gain penetration.

Finally, Cook still believes tablet computing is the future.

"I believe the tablet market will surpass the PC market," he said on the call.