It's probably all because Nvidia knows their chips can go a bit faster, but to do it, they need more voltage -> higher clocks. However, Nvidia (and most partners) won't release the standards chips like that, because they'd be well over 300watts, and outside PCI-e spec.
If this is the case, it's not good news for gamers at all. We have standards for very good reasons, and having GPUs released that are dependant on overclocking, especially over-volting isn't a good trend.