Twitter bootstrap - nested lines versus multiple lines

I started using Twitter downloads for this application I'm working on.

I read the documentation on nested rows in both a fixed grid and a liquid grid.

Now i want to do something like this

So, of course, I could do something like this

<div class="container">
    <div class="row">
        <div class="span 12">red</div>
        <div class="row">
            <div class="span 3">yellow</div>
            <div class="span 9">green</div>
        </div>
    </div>
</div>

and I think that I will get what I want. But I wonder what the consequences are.

<div class="container">
    <div class="row">
        <div class="span 12">red</div>
    </div>
    <div class="row">
        <div class="span 3">yellow</div>
        <div class="span 9">green</div>
    </div>
</div>

Now I don’t see any difference in my browser, but I am wondering what will happen if I include several line elements in one tag container. Is row-nesting the only right way to create something like what I showed? What is the difference between these two implementations of my design, so to speak?