The reason for this is simply that the raw-values and computed-values sections of your spreadsheet may grow quite large as you build it. The reference values, on the other hand, tend to be small in number and stay that way.
If your reference values are contained within the upper left corner, you can freely expand the more dynamic portions of your spreadsheet, and add and delete columns and rows, without pushing your reference values farther and farther outward. You'll always know where to find them.