That is why the video is titled formation attempt .
This stuff is extremely hard. A thousand different variables are coming into play while trying to hold formation. Wind, p factor, mixture setting,the list goes on.... hundreds of things have an effect in real life flight, and Flight Simulator does a great job of recreating about 95% of them.
I would say formation flying in online FSX is probably the most challenging thing I have ever done in a video game.
You will notice this was formation attempt #1. There will be a #2.
Eventually, we will be in such a tight formation that we could reach out and touch the other ones plane.