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?