Yeah the event is a bit complicated so it's not easy for them to explain it in the limit space in game.
1st bond you open is 25% of the guaranteed champ
2nd bond you open is 50% chance of the guaranteed champ
3rd bond you open will give you a the guaranteed champ 100% of the time
So if you get bad luck on the first 2 you will always get the guaranteed champ from the event on the last / 3rd bond.
Also as well as your chance on the guaranteed event, if you fail to get the guaranteed champ, you have your normal chance of getting a Grade V when you open a bond. So if you don't get the guaranteed champ on the first or second try, you can still get other Grade V's. The other Grade V's will come at rank 1, 2 , 3 as normal - the only champ that comes at rank 5 lvl 30 is the guaranteed one.