Your FTP is not your best 20m AP.
Most/all of the 20m FTP tests are designed to pre-fatigue you in a way such that the 20m power times a multiplier closely resembles your max 60m power.
Edit: Stick with your previous hill method (though probably not ideal). A random 20m best should/will always be higher than a "test" that is meant to mimic a 60m effort.
Last edited by ancker; 10-04-18 at 09:56 AM.
Reason: More stuff, yo.