So i was playing a match and i hit the jett for 154. Isn't the max damage 150 ? Also sometimes it shows I gave 300 damage. So what does it mean?

I just hit Plat for the first time and I am trying to improve my general gameplay and game sense step by step. One question I still can not really answer.Is it better to peek long angles or is it better to PreAim/Hold Angles? PreAim/Hold...

​ [Excuse the lack of aim. I really liked the way I was able to pull off a crazy 1v6 ace. I was low hp which made it even better! ](https://reddit.com/link/kb6d1r/video/cerj979i7l461/player)