Please note that official support for this menu version has now ceased. There are still plenty of users, though, and the forum is still running. Some of our long-time users may be able to help you out.
In "guess mode" here, but I think the white strip is from the background image you are using for the main menu strip. win_xp.gif is not a solid color item, but actually has 4 or 5 shades in it. You have to blow that puppy up to 600% or better to see it, as the changes are subtle. The end of that one menu just happens to hit a color change in the background (I think - but then I'm old and tired and hungry, so who knows?).
Even old, tired, and hungry, your guesses are pretty sharp John! Indeed, you are correct sir. (Note, however, that the offending image is actually winxp_back.gif). I think that it also has to do with the margin settings in the obscure "miscellaneous" properties field of the menu array. In any case, if you simply remove the "winxp_back.gif" reference in the 19th element of the "XPMain" menu (leaving that element blank), the white strip disappears. It's unclear to me why there's a background image specified for the Main menu anyway, since that option only works when the background color is set to transparent, so that the image can show through. The mouse off background color for XPMain is explicitly set to #EFEDDE, both in XPMain's style array (XPMainStyle), as well as in the alternate properties (offbackcolor) of three of the four menu selections (it's not set in the first one, so the style setting is used). So, the background image would not show through anyway... except for that pesky white strip!
What seems odd to me is that you get the "white strip" effect on the last menu item, no matter how many items there are. Odder still is that the sample on the website works... only real difference I can see is that the menu array script in that sample is actually embedded in the html. The reference there is to /menuimages/winxp_back.gif, but that file does exist, and as far as I can tell is exactly the same as the one in the downloadable example. So I don't know what gives there.
Anyway, to sum up my own old, tired, and hungry rambling, getting rid of "winxp_back.gif" in the 19th element of "XPMain" menu array seems to get rid of the white strip.