Apple designs their own image signal processor and their own video encoder. These are hardware based on the SoC (system on a chip). So the focus is faster, the low light filtering is more precise, the removal of noise, reproduction of color….all done onboard the A8 processor
Couple this with the fact that core image is available to developers for incorporating their creativity, and Apple know exactly how much to put in the resource when the camera is running…the result is image quality superiority.
Even if a competing hardware platform…say HTC. Samsung or even Sony were to also use hardware video and image processing and encoding, their processor is made by Qualcomm…which isn’t a bad thing, but it isn’t made for HTC camera image processing, and it isn’t made for Sony Video encoding.
The Apple A8 is purposefully and exactly built for iPhone image processing and video encoding.
It’s that simple.