なぜ `float:right` は HTML のスパンの順序を逆にするのでしょうか?

<code class="html"><div>
    <span class="label"><a href="/index/1">Bookmix Offline</a></span>
    <span class="button"><a href="/settings/">Settings</a></span>
    <span class="button"><a href="/export_all/">Export</a></span>
    <span class="button"><a href="/import/">Import</a></span>
</div></code>

<code class="css">span.button {
    float: right;
}

span.label {
    margin-left: 30px;
}</code>

<code class="css">span.button:nth-child(1) {
    float: right;
}

span.button:nth-child(2) {
    float: right;
    margin-right: 5px;
}

span.button:nth-child(3) {
    float: right;
    margin-right: 10px;
}

span.label {
    margin-left: 30px;
}</code>