'scale-to-fit' in both directions

Given this XSL-FO fragment:

   <fo:external-graphic
     border="solid 1pt"
     content-height="scale-to-fit"
     content-width="scale-to-fit"
     height="2in"
     width="5in"
     src="url(images/flower1.jpeg)"
   />

I would have expected the content size to match exactly the viewport size, 
possibly leading to non-uniform scaling. It seems that XEP has a different 
interpretation, as it preserves the original aspect ratio (see attached 
PDF). Is that the correct behaviour? (This is the result I would expect with 
'content-width' set to 'auto' instead of 'scale-to-fit').

I'd be very grateful if someone could enlighten me on this issue.

Thanks in advance,

   JYB