I asked the around the Community Manager team and it turns out Area 51 proposals were not categorized when this site was proposed. So when categories were introduced, Robert Cartaino went through all of the current sites, one by one, and picked out the categories based on what the best fit appeared to be for that site.
Site categories are not identified by the subject matter of a site - they are more representative of the target audience of the site. Quantitative finance is a science in terms of what kind of practice it is. However, the experts and the curious in the field are far more likely to be found in the realm of banks and financial companies. It is a science that is exercised and applied in the world of business. As such, it is grouped into Business due to its placement in that world, rather than being grouped with Science due to the content of the topic.
We are considering changes to the categories, but there's nothing to report yet. A good scheme would be sensible for both current and future sites. Alongside this we'll also be looking into how to better address the bottom bar so that Business isn't wholly excluded. For now, though, space concerns mean that it won't be shown.