Are some songs on HDTracks different mixes?

Has anyone noticed that some of the HDTracks songs are mixed much different than the redbook version? Specifically on the Seal Best 1991 - 2004 album at the 4:32 mark of the song 'Don't Cry', the backup vocals are almost non-existent. It sounded as if I was missing a channel so at first I thought it was my equipment or my Pure Music player software. I then played the redbook version and the backup vocals re-appeared. I noticed some other slight differences in some other Seal songs on this HDTracks album. Does anyone know if this is this common to have differenct mixes on HDTracks songs compared to the redbook versions?
Don't know how common this is, but I had a similar experience on Steely Dan's Two Against Nature (96kHz FLAC)-- the female background vocals were very faint, and most of the horns were also. Other parts sounded fine. On the other hand, Steely Dan's Gaucho (96kHz) sounded fine to me in all respects.