Fleet is definitely not an average of ship experience. Although it may be the sum of the experiences of the fleet divided by the number of ships at the time the experience was gained.
Take an experienced fleet, add a base to the fleet, then remove all the ships. The base now gets the 50% legendary fleet bonus, and even though it's just an orbital factory, enemies have a -50% chance to hit it
Try giving your enemy a stack of unarmed fighters, then attacking with PD ships to gain experience
with a legendary ship & fleet, you can retrofit to nearly invulnerable beamer ships.