It's a conceptual rhyme.

Here's a limerick following the same principle. (I think I read it in Metamagical Themas; not sure if Hofstadter was the original author.)

There once was a man from St. Bees
Who was stung on the arm by a wasp
When asked "Does it hurt?"
He replied "Just a bit —
I'm so glad that it wasn't a hornet."