The curve represents the math of the sound going down gradually:
https://www.phys.unsw.edu.au/jw/dB.html
Logarithmic. The software engineers apparently wanted the curve to be represented in the interface in a mathematically accurate way.
The way you perceive it is a straight line (linear) – full volume to silent. Has to do with the way we perceive things. F-Stops with their odd numbers – also caused by the way we perceive light, similar to sound, logarithmic, blah, blah, blah –
https://photography-on-the.net/forum/showthread.php?t=71897
In the end listen to the fade and decide if is sounds good to you.
If it does not sound good or right to you:
Look up “record audio keyframes” in your manual and do the fade with the mixer letting FCP add keyframes for you. This way you can tweak the fade to sound just the way you want it to.
If you don’t want to crack the manual here is a link for you:
https://www.geniusdv.com/news_and_tutorials/2008/06/recording-audio-keyframes-in-realtime-in-final-cut.php
Alternatively add points manually in the timeline and tweak until it sounds right to you. Just ignore the way the curve is shaped and listen.
Best,
Tom