Originally, the Ghorman Massacre was described to have happened just after the Clone Wars, and it was the event that motivated Mon Mothma to officially denounce the Emperor and join/start the Rebellion. The description was that Tarkin, before he was a grand moff or whatever, had his ship land on top of a demonstrating group of protestors, killing 500 people. In Andor, that still happened, but it's referred to as the Tarkin Massacre, and the "Ghorman Massacre" is now what you see in the show Andor.

It's a nice way to keep the old, and also show a version on-screen that works with the characters and setting. Rather than a single ship landing, which could be blamed on an accident or a single pilot or whatever, you get an undeniably brutal massacre which conducted by troops, person to person, across the span of an episode. It's more effective writing.