The iPad 4 wasn't supposed to come out until March/April, one calendar year approximately after the iPad 3. But because of the lightning connector, they decided to put the A6X out, reuse the iPad 3 form factor and screens and just keep the cables consistent. That and the technology to make an iPad as thin as the Mini was likely not ready. The A6X is a powerful chip. Putting it on a smaller motherboard takes time.
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