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.

What they said. Many old records from the golden age of vinyl sound way better than anything recorded now. And many originals sound better than repressings. My feeling is that once any recording is digitized for remastering, the essential essence of the original is gone.
Check out Acoustic Sounds for reissues.