Why does the "du" command show a smaller value for email disk usage than the cPanel interface reports?

Question

Why does the "du" command show a smaller value for email disk usage than the cPanel interface reports?

Answer 

When the setting "Compress Messages" is enabled in "Mailserver Configuration" in WHM, the size of the messages on disk is a smaller, compressed size, while the amount reported by cPanel remains the full, uncompressed size.

Additional resources

Mailserver Configuration