Why then are "Apple DOS 400K/800K" disks mentioned here as being compatible? As you know, the only way to get 400K or 800K onto single sided 3.5" media is to use a variable speed drive mechanism, which is exactly what the Macintosh 128K and 512K used.
There are actually THREE different ways to read/write variable bitrate data.
1: Change the rate at which the disk spins. The Apple 3.5" drives do this.
2: Change the rate at which the hardware reads/write bits. The Central Point Transcopy card does this, and uses IBM PC 3.5" floppy drives that spin at a normal constant rate.
3: Handle the rate differences in software. This is what the Kryoflux does. It just reads and writes flux transition information sampled at a very high resolution (higher than the bitrate). Once read, the software can then analyze the signal and decode data regardless of the bitrate. This also lets it do all this on normal IBM PC drives.
The issue with IBM PC style drives is that to improve reliably they filter out noise and amplify weak signals. Unfortunately, these are optimized around standard bitrate MFM encoded disks used by IBM PC compatible computers. That means GCR encoding and odd bitrates may look like "noise", and wind up getting filtered out or distorted.
Different drive models, and even the same model with different revision hardware, may implement this slightly differently and yeild different results. As mentioned, add age, usage, and other variances in the mix, and two seemingly identical drives may give different results.
Heck, even when dealing with damaged or marginal standard IBM PC floppy disks, I sometimes have to try different drives, and may get different results.