CD quality differences

You are not imaging things! There is considerable variability in sound quality across discs. I hear it all the time.

As for different quality within a disc, I've not really experienced this unless an artist adds a very old recording to the mix for prosperity.

HD video is the same with lots of variability. It's just something that comes with these entertaiment products.

Listen to "Sledgehammer" and "Big Time" from Peter Gabriel's "So". There's no doubting these cuts were recorded, mixed, or mastered differently than the other songs.

Different studios, instruments, recording equipment, and musicians are also contributing factors to what you are hearing.

I've noticed this difference most on anthology or greatest hits CDs. The earlier stuff sounding harsh and nasty (probably early solid state recordings) while the more recent stuff starts to sound better. Recently heard this on a Roxy Music CD and a Bruce Springsteen CD. The difference from beginning to end is quite dramatic on both CDs.

Experienced this recently while listening to "East West" by the Paul Butterfield Blues band. "Never Say No" is a markedly superior recording to the other tracks on this generally well-recorded album.

There is a thread on here about better remasters of some Stones CD's.  I think it is the 2002 abkco one.  I am replacing my 1980s Stones CDs with those.

But note that these are all based on alterations in the studio of the sound from the original masters, not on whether the CD spec. is deficient in some way.  There is a lot of smoke and noise from those claiming the latter is not adequate, but no double blind listening tests show higher bit rates, 1-bit etc. etc. sound better.