Why does my flexbox expand despite overflow:hidden/auto?

Here's one way to do it: http://jsfiddle.net/1ejgybmv/.

* {
    padding: 0px;
    margin: 0px;
}

html, body {
    height: 100%;
}

body {
    display: flex;
    flex-direction: column;
    background: lightblue;
}
#top {
    flex: 1 1 auto;
    overflow-y: auto;
}

#bottom {
    flex: 0 0 1em;
}

li {
    height: 2em;
    line-height: 2em;
    background: white;
}