Update _top-bar.scss

The final padding unit is in rem and you add 20 (supposed to be px) to it. The expected results should be $topbar-link-padding (in rem) + rem-calc(20).

I did two screenshot to show you the issue: http://www.pixentral.com/show.php?picture=1FFZD61cybeKURII6jZa2cMgq8NCnr is the actual zurb release, and http://www.pixentral.com/show.php?picture=1d8mmkTfQcA0QeTdKlTdvEaWfpMr1 with + rem-calc(20) fix