Fluid Grid Too Wide in Firefox

I've encountered an issue where the width of spans in `row-fluid` elements add up to over 100%.  I see that in the `grid-fluid-span` mixin, there are two widths, one normal, and the other prefixed with an asterisk.  The one with the asterisk (which Firefox ignores) has the correct mathematical function and adds up to below 100%, but the first one adds up to too much.  Why is this happening?
