Is Disturbed music clified as explicit?

I am a huge fan of Disturbed, but I noticed that two of their albums are marked as explicit (parental advisory): The Sickness and Believe. (Believe's notification is on the back.) I own Ten Thousand Fists and Indestructible. Indestructible uses the "F" word a few times (I think four), yet it does not have a parental advisory anywhere. Furthermore, according to iTunes, Disturbed only has one explicit song: Down With The Sickness. If that is their only explicit song, why didn't they just have it as a single so their album could be clean, and why is the album Believe marked as explicit?