Basically, after the chips are manufactured, they're not 100% uniform. Some have better performance, some have worse.

In this case, it means they're reserving the best chips for the founders cards. In other cases, there have been instances where a company has two products, a high end and a low end (or medium, etc). In some of those cases, people have investigated and the chips are actually exactly the same, but the lower end product will have a core disabled or similar, depending on the exact product. That'll happen a lot of the time when the company has yield issues where too many of the chips don't have acceptable performance or one part of the chip is just broken. They'll disable the broken portion and boom, the lower end product is born. That's still a net win for them because the alternate is either to throw the entire thing away or spend more time improving the yield.

chip manufacturing is an imperfect process, and so there is variance in performance/viability of all of the hardware on a chip. the higher performing chips are "binned" for the top end of the price point, while the lower performing chips are either binned for lower performance or have some of their functionality disabled. For contrived example, A company may produce nothing but quad core chips but sell those with some cores that don't meet minimum performance as dual cores with the bad cores physically disabled.

they cherry-pick their best silicon so you can run them at higher speeds if you're overclocking or lower voltage if you want cooler temperatures and less power consumption at stock speeds.