Reason being is that if you use fib retracement on the previous bull runs, the next that occured hits or surpasses the last level of the fib.
As you can see on the chart we haven't topped it yet on this bull run. My guess is that the top will be around at 74.300$ that's ~55% from the current price.
Then it should end this BTC SEASON and then starting the ALTCOIN SEASON!
Now I never said that it has always to hit the last level! This is just what happened before.