When Apple comes out with a new iPad, the company often discounts previous versions. But there's no option to get the third-generation iPad at the Apple store; Apple discontinued the third-generation iPad last week. Officially, the fourth-generation iPad is called the "iPad with Retina display," even though the third-generation iPad initially introduced Retina to the tablet.

Apple, of course, likely knew the third-generation iPad would be short-lived. In Apple's second-quarter earnings call this year, shortly after unveiling the third-generation iPad, the company reported 2 million iPads in channel inventory, below the target range of four to six weeks.

"Part of the difference in [Apple's] third-quarter sales, we believe, is that they were actually scaling back inventory on hand of the new product, so that they could prepare to launch the next product," says analyst Rhoda Alexander at IHS iSuppli.

It's as if the third-generation iPad never existed.

Technically, Apple needed to get to the fourth-generation iPad. The third-generation iPad powered the high-performance Retina display with an A5X processor, but the fourth-generation iPad does it better with a dual-core A6X processor and quad-core graphics.