Impact of td/th background over table border

I have a table element with a border:

.summary-table {
    font-family: sans-serif;
    -webkit-font-smoothing: antialiased;
    font-size: 115%;
    overflow: auto;
    width: 100%;
    border: 1px solid rgb(195, 195, 195) !important;
    border-radius: 4px;
    overflow: hidden;
}

When i use background color on thead tr it works fine

.summary-table thead tr{
    background-color: #E8E8E8 !important;
}

But when i use background color on thead th i get this (no border-top and left ):

.summary-table th {
    background-color: #E8E8E8 !important;
    font-weight: normal;
    color: black;
    padding: 20px 30px;
}

Can anyone tell nme why i get this?

via Chebli Mohamed