Floppy disks again

Okay, one final question before I drop this topic forever (or until tomorrow, whichever comes first). How is it possible that DD media could be of such poor quality that it can't (reliably?) do 96 TPI, while still being just fine at 48 TPI? I mean, it is recording something like 3000 bits per inch along each track, right? How could that same media not have enough resolution to keep the bits of adjacent tracks separate at less than 100 per inch? Can somebody who believes this can happen give me a mental model of what is going on there? I mean, in terms of physics or geometry or mechanics or anything measurable and specific. It sounds to me like it must be the drive, and not the media, that limits the number of tracks to anything below 1500 TPI. Thanks, Bill.