How does a hard drive retain information after being formatted?

Companies try to zero-fill their hard drives several times when they decommission them, why? Surely one pass of zero-fill would do the job exactly as they need. I understand the reasoning for the companies to do it but the need to run it several times. Could someone explain the process to help me with my understanding.

Physics. Rewriting the magnetic domains on the platter weakens them, but still leaves a residual amount of magnetism. The level of magnetism drops below the threshold to be readable by the electronics in the drive, but it is easily detectable with the appropriate equipment.

Here is a great article (and comments/links) about disk wiping, single vs multiple passes