Astonishingly, this discrepancy has never been explained1. No canon source addresses it, and neither has any Legends source (so far as I know). There was much fan speculation that this would finally be answered in Rogue One but, alas, it was not so.
Amusingly, the original Death Star plans, seen in Attack of the Clones, depict the focusing dish in the correct, angled position:
So it can't even be explained away by being an early design idea that was later abandoned.
It's because the animator was working based on an early Ralph McQuarrie painting of the Death Star, which depicts the dish at the equator2:
The animator, Larry Cuba, discusses the design process (and mentions this fact) in a video he produced on the subject:
1 Except the once
2 Full disclosure: I didn't actually know this. I found it out reading Pablo Hidalgo's Twitter feed, which is fascinating.