The lyrics of Jingle Bell Rock goes:
>Jingle bell, jingle bell, jingle bell rock  
Jingle bells swing and jingle bells ring...

I wonder why the '**bell**' in '_**Jingle bell rock**_' is singular yet it's '**rock**' and not '**rocks**'.  
And then why is '_Jingle **bells swing**_' not '_Jingle **bell ring**_' or '_Jingle **bell rings**_'?  
Any reason why are the singularity and plurality inconsistent? Thank you!