Here's what happens:
* Cato is already injured: Cato is already wounded by the time Katniss shoots him. He has been fighting with Peeta and has suffered multiple injuries. This weakens him and likely reduces the impact of Katniss' shot.
* The arrow hits his shoulder: Katniss aims for his shoulder, not a vital organ. This ensures he doesn't die immediately, but it's still a significant injury.
* Cato is focused on Peeta: At the moment Katniss shoots, Cato is focused on fighting Peeta. He is momentarily distracted, perhaps not noticing the arrow until it hits him.
Although Katniss' arrow doesn't kill Cato, it does weaken him considerably. This allows Peeta to gain the upper hand and eventually finishes him off with a knife.
It's important to remember that even though Katniss' arrow might not have been a fatal blow, it was still a significant factor in Cato's demise. It weakened him and gave Peeta the opportunity to defeat him.