About the 8 frames I'm surprised everyone gives spooky's explanation credit because it doesn't make total sense.
Harada
explained that Tekken's netcode is removing frames to sync both players, but it removes them INSIDE the move, not before it. If a move has 10 frames of startup and you need it to come out 3 frames earlier because you have 3 frames of lag, the move will come out in 7 frames, deleting startup frames.
It's basically a rollback done with a delay netcode that isn't visible for the player. There isn't more or less input lag, the move always starts at the right time after you press.
Since SF5 has moves starting in 3 frames, Capcom can't do this startup frames delete method. You have to check the startup and come back, exactly how the rollback works today.
About the 8 frames is that it's vsync related. If you remove vsync on PC, you get your traditional 5 frames of input lag, just like on consoles. But on PC, SF4 had more lag than consoles because vsync was activated by default. All PC players removed it to match the console version delay.
My guess is: since they didn't want to game to have tearing, that they needed a similar input lag both on console and PC, and considering that vsync can probably change depending on you computer/Drivers, they choose to keep it and added frames dedicated to vsync rendering. And after that, they built the gameplay with this problem in mind.
Also you probably don't know this (and I can talk about it now I guess) but Capcom started to build SF5 on their own engine. They moved to UE4 at some point in development. I wouldn't be surprised if all this is coming from Unreal Engine.