Classical tracks, like all others, are provided by the publisher, who is responsible for digitizing the content. Unfortunately, some publishers don't seem to take much care.I have bought only one classical track through iTunes, and will never buy another. The sound quality was terrible. Maybe some are better than others?
For older recordings that may have not been digitally remastered, you might be getting a file digitized from the vinyl or cassette master or (if the master is no longer available) a pressing. These will definitely not sound as good as something where the publisher has pristine quality multi-track recordings and can perform a proper remaster/remix.