So according to the John Lennon webpage, and now it makes sense why the Imagine tracks do have a different sound quality and mix:
“Mixed and engineered by multi GRAMMY® Award-winning engineer Paul Hicks, who also helmed the mixes for 2018’s universally acclaimed
Imagine – The Ultimate Collection series, with assistance by engineer Sam Gannon who also worked on that release, the songs were completely remixed from scratch, using brand new transfers of the original multi-tracks, cleaned up to the highest possible sonic quality.
After weeks of painstaking preparation, the final mixes and effects were completed using only vintage analog equipment and effects at Henson Recording Studios in Los Angeles (formerly A&M Studios, where John had recorded some of the Rock ‘n’ Roll album), and then mastered in analog at Abbey Road Studiosby Alex Wharton in order to ensure the most beautiful and authentic sound quality possible.”