Complete and random fluctuations in power. Best example is DBZ - Vegeta can blow up a planet using a move he doesn't have to charge up when he has a power level of what was it, 6000? Something like that at least (Less than NINE THOUSAND for sure).

Skip ahead a long ways, and you've got people with power levels in the billions, who charge up moves for five minutes/thirteen episodes, and don't even blow up an island.