Font used in footnote popup

This is an AZW3 file I've created.
Could you explain why the footnote has a different font (noserif) than the main text?
I haven't specified a font in the css. The main text uses the user's choice.

Here's my definition for the footnote – but it's not really relevant as it is how the footnotes appear on their own page, not in the footnote popup:

.xxnote {
display: block;
font-size: 90%;
text-align: justify;
text-indent: 0;
margin: 3pt 0 0;
}

Thank you.

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