Ariana Grande Explosion Manchester another False Flag attack?

Just some months ago a terrordrill was conducted at the venue which now has been hit by an explosion. Not only the same venue but also the same scenario, an attack in the foyer of the venue. One must wonder what the chance of this is that a venue is being targeted by terrorists after a drill with the same scenario has been conducted on that very same grounds? RT showed us these drills. See it on:

These drills seem to come before the actual event. We saw this with the London Bridge bombing, the Paris bombing, the Boston Marathon bombing and several other would be terrorist atracks. I'm not a mathmatician but can someone give the calculations what the chances are that you will have an attack and there's shortly been a drill by the same lines as the following attack?

And what do these drills as they are not succesfull in stoping the attack or minimizing casualties? Is it nowadays that once you know of a terrordrill that you have to avpid these places because you can expect the true scenario to happen? The same happened in Sweden.

Already during the attacks of 9/11 there were several drills that day with the same scenario as happening the same in real time. The question is which terrorist can order such a drill? And afterwards we never seem to hear from it in mainstream media as being odd in the whole line of events? What tells the occurence of a drill about the possibility of it being a false flag attack?

I don't know, as we all don't, what happened here. It strikes me as odd that the two phenomae, the drill and the attack seem to be coexistent and that raises the question if there is more to this then meets the eye. Is this another false flag attack?