This is a cool lesson in genetics, for you! You can learn a lot about biology, just looking at the plants in your garden.
For most plants grown from seed, even the most inbred commercial lines, there will always be some plant-to-plant variation. Many plants tend to self-fertilise, meaning they produce pretty uniform offspring, but even then - if they make flowers, they'll occasionally mix up their genes. It's more or less impossible to make plants completely uniform, without some crazy genetic tricks, or cloning (through, say, cuttings or grafting).
Most often, when an extreme cultivated plant (like your sunflower here) shows variation, it'll be more like the wild type. If you look at pictures of wild sunflowers, you'll see that they're much smaller than cultivated ones, with small seeds (too small for humans, perfect for birds). Typically they'll be bright yellow, and will have a lot of branches, and lots of flowers. We made them single-flowered, because like Megan said, it ensures big, easy to harvest seeds. So if that's what you want, then chop off all the side branches.
In our garden, over time, we just let the sunflowers seed themselves for a few years, and now they all look like the wild ones.