I've only seen a few episodes (and half a film) of Highlander, and I never really understood the Quickening:

The Quickening is the receiving of all the power and knowledge another immortal has obtained throughout his/her life.

If that were true, there would be enormous differences in power between two immortals (an old immortal being a thousand time more powerful than a new one for instance).

But it seems to me that all of them are very close in power. How is that possible?