In space, orientation is relative
Unless the Death Star is very close to a planet or other massive body, any significant gravity it produces is artificial. As such, whether the laser dish is in the southern or northern hemispheres is purely a matter of camera angle.
Sometimes it looks like this:
Other times it looks more like this:
But in either case, the laser disk is in the same position relative to the ship itself.
Many shots are taken from the point of view of the ships flying around the Death Star, so that probably influences what the apparent orientation is, as is probably the case in the second picture.
So another answer might be:
In order to give the impression of flying around the Death Star