Not sure, maybe they are broken in after time?
I can't say I think they sound better, but they sound the same to me. I have bought a few used ones and they don't sound as good as the ones I originally bought. I think most people in general do not take very good care of their records. I use MFSL sleeves and several years ago started using jacket sleeves (not nearly as important) and a month or so ago got box sleeves for my UHQRs and a Kind of Blue box set. (Even less important since they already have the box, inner spongy packaging sheets, inner jackets, and record protectors). Outer boxes were getting worn.