I believe it's a javascript load hiccup in the IE6 engine that XWD preview uses.
I have noticed slight anomalies like this in the past with background positioning in the preview window. Don't rely on this as the best previewing environment.
XWD's HTML export is OK and it's fine in your regular browser.
Please export this to HTM and test in IE8 or FF for example.
My sample only uses tweaks from the Tweak set.
Also, please you could attach a .web file for us to look at your problem?
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