ORIGINAL: DarZeelon
Or the sleeve could have become warped and is no longer round.
Aluminium pistons don't permanently 'grow' as meehanite (B grade cast-iron) ones do, from heat treating.
...Neither can brass sleeves shrink.
Becoming 'out of round' is the only 'wrong' dimensional change that can occur; beside galling.
That's what I think too Dar. Ask anyone who has overheated a Chevy Vega Four (oval) cylinder. My guess it's just the manufacturing "luck of the draw".
Bill