It's explained by Niels Ferguson and Bruce Schneier in Practical Cryptography in section 22.10.2, "Magnetic Storage".

Overwriting does not completely destroy old data. You can think of it as repainting a wall with a single coat of paint. You can still vaguely see the old coat of paint under it. The magnetic domains can also migrate away from the read/write head either to the side of the track or deeper down into the magnetic material, where they can linger for a longer time. Overwritten data is typically not recoverable with the normal read/write head, but an attacker who takes apart a disk drive and uses specialized equipment might be able to retrieve some or all of the old data.

They also advocate multiple, random overwrites using fresh data as a best practice at this time.

It seems like that should get harder and harder as the density of disks increases. I wonder how different the ease of recovery is for drives made now compared to when that book was written.