Here's why:

* Magnitude is a measure of brightness, but it's an inverse scale. This means a lower number indicates a brighter object.

* The magnitude system originates from the ancient Greeks. They categorized stars by their apparent brightness, with the brightest stars being assigned a magnitude of 1. Dimmer stars were assigned higher numbers.

* The system was later refined using scientific measurements. However, the convention of lower numbers for brighter objects remained.

So, while it might seem counterintuitive, a star with a magnitude of -1 is brighter than a star with a magnitude of 1.