Hmm... Thats sounds like something interesting. Also, it doesn't necessarily have to be a chip that didn't perform well in other cases. Think about it, if there is demand for 2000 2.0Ghz cpus, and 1000 chips demand for 2.6Ghz chips, and those are the only two they sell, and Intel makes 1500 that perform at a stock 2.6 speed, why waste them and keep making more to get ones that don't perform that to use as 2.0ghz cpus? They'll just use those as 2.0Ghz chips. Also, they keep enough headroom for high ambient temperatures and voltage fluctuations so most chips should overclock decently...
One more thing, if there highest model is 2.4Ghz for a Celeron, what do they do with the ones that perform at a much higher speed such as 3.0Ghz? They'll just use that as a 2.4GHz cpu.
See, it doesn't have to be a chip that didn't perform as well as a higher chip.