Problem with background color on horizontal menu

Hi!

I am having a problem getting the background hover color of the menu buttons to stretch all the way down.

In this picture it is fine:

But on some operating systems, the light gray isn't stretching all the way down.

I though me using px for the padding on "#jsddm li a" was causing it. I changed to em, but the problem still exist on some browsers/operating systems.
here is the css I'm using:

#jsddm {
    margin: 0;
    padding: 0;
    text-transform: uppercase;
}

#jsddm li {	
    float: left;
    list-style: none;
    font: 1em helvetica;
    font-weight: bold;
}

#jsddm li a { 
    display: block;
    padding: 0.35em 0.6em 0.4em 0.6em;
    text-decoration: none;
    color: #ebebeb;
    white-space: nowrap;
    background-color: #464646;
    outline: 0px;
}

#jsddm li a:hover{
    background: #636363;
    cursor: pointer;
}

#jsddm li ul { 
    margin: 0;
    padding: 0;
    position: absolute;
    visibility: hidden;
}

#jsddm li ul li {
    float: none;
    display: inline
}

#jsddm li ul li a {
    width: 107px;
    font-size: 11px;
    border-top: 1px dotted #636363;
}

#jsddm li ul li a:hover {
    background-color: #636363;
}