Usually i only see this behavior happen if you edit the width of a menu item directly in the css rather than using the wizard.
When you edit the width in menu, are you using the option in the property inspector to "Edit Menu" or "Edit Theme"?
Use the "Edit Menu" Option, then click the Define Button next to theme to edit the theme and change the width of your menu items. Sometimes, if the Menu contents change (especially if a new level is added) between editing the theme, using the edit theme option wont properly take the new level into account.
using the "Edit Menu" option causes the wizard to reconcile the theme against the contents.
the true width of an object using css is calculated as left padding + width + right padding
in your menu, you have a 5 pixel padding set for top, right, bottom and left of each menu item, meaning the true width of the menu items is 190px.