Old vs. new

In the old days of vinyl, analogue source tapes were mixed down to a master tape. Duplicates of this master were sent to a pressing plant where the records were manufactured.

Today to reissue an album, a digital file is created from the analogue tape and becomes the new master. But to do this, some digital processing and compression takes place, thus changing some of the characteristics of the analogue signal. This file is used to create both vinyl and CDs, although vinyl which is an analogue medium requires different specs than the digital product.
The new file has the proper specs required by the pressing plant, but the sound is no longer identical to the original master due to digital processing.

The quality of these digital masters are of varying degrees of quality depending on the engineer, producer, and record label.
 
The file is sent to the record plant
where the pressing process is the same as before.

As roberjerman was eluding to, these reissues which are now digital come from old analogue tapes which may have deteriorated, or even been lost. So whatever tapes they can find are used to release the album on new vinyl.
Of course, the quality of these tapes will never equal the original masters from years gone by.

Cooder heard about the new recording format, digital, and, being a fanatic about the recorded sound of his incredible guitar tone (he's a master), gave it a try on his Bop Til You Drop album. He hated digital! Back to analog he ran

That's too funny. There were many stories like that in the early days.

That's interesting about Bop till you Drop in '79. But very few rock bands were recording to digital tape until the mid to late 80's. Studios were slow to transition to digital (very expensive) and most bands didn't have the budgets to use the new technology. Some music was being mastered to digital tape, but recording was analogue. The proof is in all the CD's marked ADD.

Classical music producers went all-in with digital recording about 1979 or 80. And artists like Michael Jackson were early adopters of digital.