In general without the scientific explanation, airgun pellets stabilize a little differently than powder burning projectiles.
Pellets rely on their diabolo shape (somewhat like a badminton birdie) to keep their nose forward. Yes the rifling imparts spin and this increases their accuracy but the wasp waist shape design creates a aerodynamic drag that keeps the nose out in front...this drag (in projectile terms) slows it relatively quickly....this is usually always a bad thing for shooters, because it is the time of flight that determines how flat a trajectory is....(why we love slippery smooth fast spinning long boatail bullets to reach out there.)
When any projectile crosses (decelerates) back down below the speed of sound, very turbulent drag forces act on it in ways that disturb it slightly from its supersonic flight path...creating inaccuracies...(22 lr can be super accurate out to about 75yds where the bullet becomes subsonic...that turbulence changes its flight path and groups open up.
That turbulence applied to a pellet projectile that is relying on air drag as part of its stabilization would be somewhat doomed as it traveled supersonically and then crossed below the sound barrier.
Hope this helps you understand...airdrag and ballistics science is PHD and mega math territory...
Thinking it's the transonic are they are trying to avoid. Starting super-sonic and then transitioning to sub sonic between muzzle and target seems to do "odd carp" to the projectile. So either keep it sub-sonic all the way, or keep it super-sonic all the way.
With airguns, and the poor BC's of pellets, really don'rt have much of a chance of keeping it supersonic all the way to the target, so we tend to keep them sub-sonic.
Going from subsonic to superSonic upsets all projectiles as does going back through the sound barrier as the projectile slows down. That is why long range shooters choose a cartridge and bullet that stays supersonic as long as possible.
Pellets with their Diablo shape are not aeorodynamic and upset more as they pass through the sound barrier and turbelence it creates. Subsonic at short range is usually more accurate for all types of guns