I suspect Gilbert was distorting deliberately to make the rhymes cute, as Nash did later. Putting the emphasis on the wrong syllable, when it served his purpose. He (Gilbert) did it again, and with the same sound at that, in Iolanthe:

Fairy Queen:
Oh Chancellor unwary,
It's highly necessary
Your tongue to teach
Respectful speech
Your attitude to vary.
Your badinage so airy,
Your manner arbitrary
Are out of place
When face to face
With an in-flu-en-tial Fairy.

Lord Chancellor:
A plague on this vagary
I'm in a nice quandary
Of hasty tone
With dames unknown
I ought to be more chary.
It seems that she's a fairy -
From Andersen's library -
And I took her for
The proprietor [sic]
Of a la-dies' se-mi-nary!