OK, so this is because you use the Menu option “desktop menu spacing” To increase the space between menu Items, but that option unfortunately adds the space to the right of the item, causing the bad centering.
Remove the option (empty the box), then use the Custom CSS Rule instead (with the value you like).
@media( min-width:768px) {
.menu-primary .wvrx-menu-container li a{padding-right:1.5em;padding-left:1.5em;}
}