Because we are discussing a back fist.
Quite simply a back fist will either come from the side and land pretty much like in that photo or it is going to come straight over the top and land vertically. Which is a manky way to throw a back fist.
Now it does not matter if your backfist looks any different provided it lands about in the same place leaving pretty much the same opportunity to counter it because when you throw a backfist the whole right side of your head is open.
I think you are getting muddled up to be honest, when we use back fist we make sure we are covered up and it's easy to do this, how you do the backfist matters a lot, we don't do it as shown in the pics above, ours is elbow up first then back fist and can come off either arm, we twist as we do it to give more power. At no time are you left open. I've never seen a back fist come over the top. In JKD I know the back fist is done first then the elbow comes up, I've tried to do it that way when I trained with them but it's ingrained in me that the elbow comes up first then the back fist, it works well because it's not a 'one' strike you follow it up quickly with another strike often an elbow strike.