We understand that sometimes you may outperform your Key Result target - that's great!
Objective success is a based on the performance of the Key Result(s) and it's weighted equally between the respective Key Results. Because of this, it's important the impact of a Key Result's progress be capped at 100% so that it doesn't falsely distort the integrity of the outcome. For instance, if you have 2 Key Results, one has 0% progress and the other has 200%, that would translate into an average of 100% and mean that your Objective has been achieved. Capping it at 100% more appropriately represents the reality of achieving one of your Key Results and shows that your Objective progressed 50%.
It's worth noting that an Objective's progress is only based on its Key Results. Initiatives are not calculated into the Objective progress.