centering only works with fixed positioning, not absolute

Question

I have a div in which I'm trying to center an image. It centers perfectly if position is set to fixed, but when position is set to absolute, the image is a few pixels too much to the right.

I can't have position set to fixed because this div scrolls in from the top when the user clicks something, so I can't have it be visible until that moment. I've never had this problem before. Can anyone tell me what's wrong?

html:

<div class="header">
    <img class="logo" src="img/navbar_title.jpg"/>
    <img class="tab" src="img/mid_tab.png" /><!-- image to be centered -->
    <div class="header_social">
        <img src="img/button_pg.jpg" />
        <img src="img/button_facebook.jpg" />
        <img src="img/button_twitter.jpg" />
    </div>
</div>

css:

.header
{
position:fixed;
top:-100px;
width:100%;
height:30px;
padding:10px;
background-color:black;
color: white;
z-index:10;
margin-top:0px;
box-shadow: gray 3px 0px 10px;
}

    .header .logo
    {
        position: absolute;
        top: 10px;
        z-index: 4;
        left: 20px;
    }
    .header .tab { /* not working right */
        position: absolute;
        top: 0;
        z-index: 4;
        left: 50%;
        transform: translateX(-50%);
        -moz-transform: translateX(-50%);
        -ms-transform: translateX(-50%);
        -webkit-transform: translateX(-50%);
    }

    .header h1
    {
    font-family: Helvetica, sans-serif;
    font-weight: bold;
    font-size:14pt;
    display:inline-block;
    line-height:30px;
    float:left;
    margin-top: 0;
    margin-left: 20px;
    letter-spacing: .05em;
    color: #303030;
    }

    .header .header_social
    {
    float:right;
    height:30px;
    line-height:30px;
    width:150px;
    }

Answer 1

This is due to box-sizing. When you position fixed, it is relative to the entire screen, period. When you position absolute, it is relative to the nearest non-static position parent. In your case, that is your div.header. Your div.header has a padding and a width 100%, so it is actually 100% + 20. Set box-sizing: border-box on your div.header and your image will move a few pixels to the left. :)