Jonathan in a Gallery you care for the image not to be STRETCHED since the thumbnail or image if stretched will be ruined.
That is where the FIT TO BOX is used for. It is specific so it keeps an image to specific boundaries without tampering with the photos quality.
If you stretch an image by defining width/height you might end with a totaly ruined photo (check it in DW, just insert an image and give it random width and height (in the html tage if u use CS4).
What you want is exactly FIT TO BOX and not to stretch your photos.
I hope you understand why.