Call me thick but I had never looked up the technology behind this and for some crazy reason I thought the cameras were part of the uprights and crossbar.
Now I've read up on it, the 7 cameras are mounted on the structure of the stadium and PREDICT where the ball is based on its flight from various angles. So it sounds like its a guessing system.
It would be nice to know where the 7 cameras are mounted at Villa Park so we can believe that all 7 cameras couldn't track the ball, though in reality it should only need 2 perhaps 3 to have tracked the ball.
I don't think we'll ever know but do know that the referees shouldn't trust the technology anymore.