Visual behavior is influenced by anything that makes you look (bottom-up processing) as well as the voluntary intent to look at something (top-down processing).
If bottom-up factors were the only ones influencing out attention, everyone would look at the world the same way regardless of what they know or what they are trying to accomplish.
Top-down factors are what complicate eye-tracking research and at the same time, what make it interesting.
Town-down processing is relies on previous experience and expectations as well as the task one is attempting to accomplish.
Top-down factors are responsible for differences in how different individuals look at the same thing.