sluggish centers

[Dave Nereson]

      Went to tune a lady's Baldwin spinet (not Acrosonic, but doesn't 
matter) and after hitting the soft pedal, most of the hammers drifted back 
very slowly.  It had changed climates and the action centers were sluggish. 
I've encountered this many times before.
    But what I don't get is:  why is it almost always remedied by shrinking 
only the hammer-butt center-pin bushings?  Don't all the other center pin 
bushings swell as well  (or flanges shrink, whatever the case may be)?
    I applied about 1:4 alcohol to water to all the hammer flange bushings, 
let it dry, and that freed them up pretty well, as it usually does.  Still 
had to re-pin a few.  But why aren't all the jacks, wippens, and dampers 
sluggish also?
    --David Nereson, RPT