Stereo upmixing quality varies because it depends on the pairing of recording and upmixer.

Evidence

When multichannel systems became available, enthusiasts saw an opportunity to improve upon the limited soundstage and spatial envelopment delivered by stereo. As has been mentioned, there are no rules for how stereo recordings are made, and there are no rules about how 2-channel stereo can be manipulated and/or augmented for upmixed delivery through 5 or 7 channels. This explains the wide range of opinions about how well stereo upmixing works. It is the combination of recording technique and upmixer that is being judged. With some combinations, the results are very pleasant. Sometimes the original stereo is preferred. — Toole, Floyd E, Sound reproduction- the acoustics and…, 863

Categorization

• DEFINE: Explains why upmixing produces such varied judged results: it is always an interaction between an unpredictable recording technique and a specific upmixer algorithm.