Aion and its derivatives

Question: You say the verses in Revelation that deal with "eternal" are from the Greek word aion or a derivative of it......and that this word applies to the life span of the person it is referring to - and not to an absolute eternity as we know it.

And then when Paul uses it (ex John 6:51 and other places) you say it shows that true eternal life comes from the gospel.

How can you use it one way in Revelation and another in other places?

Just an honest question. Thanks, Carl