The laser beam is kept on track by a sophisticated servo loop that is not the same in all drives. Pit geometry and track spacing affect amplitude and radial tracking quality and also determine another important measurement of quality, crosstalk. The focussed laser spot weakly illuminates adjacent tracks, but proper engineering avoids signal deterioration. Crosstalk measurements confirm this important quality characteristic. Each measures a different error pattern, usually related to the nature and physical size of the defect.
Some players correct errors that are uncorrectable in other drives, particularly high speed types.
Proper testing both confirms interchange, and also identifies very high quality that allows the disc to degrade gracefully when routine handling causes surface damage. The list of tests could be expanded, but it is obvious that functionality in a few drives does not establish quality. It can be done, and the cost is reasonable.