I agree it is confusing. I think it works like this...
Normally, when flying, a creature may only attack before or after it moves with its fly speed.
Spring Attack allows you to move both before and after your attacks and avoid AoOs from your target.
Flyby Attack allows a flying creature to make an attack at any point of its movement.
So, I could be wrong, but I believe you need both Flyby Attack and Spring Attack to make an attack during your flight and avoid AoOs from your target.
I could be wrong, though, as the text for Spring Attack doesn't limit it to land speed. It seems odd to me to have two feats that do essentially the same thing, though, and have one completely better than the other. Of course, Spring Attack does have more prereqs...