When one team is winning, people think it's an advantage for one side or the other.
In the case of FL/GA, the city of Jacksonville is as close to the state line as you can get. You can spit on the state of Georgia from Jacksonville. To give you an idea, there are people who live in Georgia who make the commute into Jacksonville to work.
People harp on the distance from each campus to Jacksonville, but we all know that when it comes to college sports, fans are coming from everywhere.
A Gator alumnus living in Miami is pretty much driving just as far to get to Jacksonville as a Bulldog alumnus in Atlanta. A Bulldog alumnus in Savannah has a shorter drive than a Gator alumnus in Tampa.
And as someone stated, the crowd is truly a 50/50 split unless one of the schools is convinced they will lose. In that case, they sell their tickets and don't go. Georgia had the "home crowd advantage" in 2014 as an example as Gator fans believed they were going to be beaten badly.