I think this is a case where a picture is worth a thousand words. This excellent article "Visualizing Commodore 1541 Disk Contents" [1] by Michael Steil about the Commodore 64 disk format, includes visualizations of the magnetic flux as stored on disk.
This bit is particularly relevant: https://www.pagetable.com/docs/visualize_1541/sector.png See the solid bit at the end of the sector, just before the next header? You could squeeze a few more bytes in there, but if the drive motor is just slightly too fast, it'll overwrite the next sector. That's why there's a gap, tolerance for timing variation.
Most floppy drive technologies wrote blindly, guessing where they were on the disk based on timing estimates since the controller last saw a sector header. This is also why disks needed to be "formatted". Not just in the sense of writing the file system data structures, but writing out all the sector headers. This had to be done all at once with the same drive, due to those small timing variations.
This bit is particularly relevant: https://www.pagetable.com/docs/visualize_1541/sector.png See the solid bit at the end of the sector, just before the next header? You could squeeze a few more bytes in there, but if the drive motor is just slightly too fast, it'll overwrite the next sector. That's why there's a gap, tolerance for timing variation.
Most floppy drive technologies wrote blindly, guessing where they were on the disk based on timing estimates since the controller last saw a sector header. This is also why disks needed to be "formatted". Not just in the sense of writing the file system data structures, but writing out all the sector headers. This had to be done all at once with the same drive, due to those small timing variations.
[1] https://www.pagetable.com/?p=1070