What, did they put out an announcement at the morning briefing, ‘hey everyone if you see a stranger on our secret base make sure to shoot at them but not too accurately, even if this leads to your death’.

Yes. Exactly that. These are soldiers, they follow the orders they’re given. Indeed, they’re elite soldiers. The Emperor fully expects that if he orders them to their deaths they will follow those orders to the letter and die for him.

They were ordered to let the Rebels go, while also trying to make it look like they were trying to capture them. That means shooting at them and missing, but only just missing. It means letting yourself get shot if that’s the only plausible way for those Rebels to escape. That takes incredible marksmanship and incredible discipline.

Remember Obi-wan himself stated earlier in that same movie that “only Imperial Stormtroopers are so precise” when admiring the evidence of their assault on the Jawa sandcrawler. Obi-Wan knows what he’s talking about. And that happened first, so you can’t even call it a “retcon” by any standard.

Again, this is all stuff that was explicitly explained in the movie itself.