Wedge: That's impossible, even for a computer!
Luke: It's not impossible, I used to bullseye womp rats in my T-16 back home. They're not much bigger than two meters.
There's something Dodonna is saying during this but it's indistinct cause of the focus on Luke and Wedge. I wonder what he was saying.
Practice makes perfect. Although, there are probably some varying factors, traveling at blazing speeds in a military fighter, nerves, and of course, the Force helped him aim much more accurately than A) a computer could ever possibly do and B) a person not Force-sensitive.
It's also a moral about technology, how we should not depend on it entirely, but it has its place as a useful tool. There are some things technology will have a great deal of trouble replicating. For instance, we can develop a computer that can beat the world's greatest chess players but not one that can even play at a competitive level with your average-skilled
Go player.
It all boils down to a storytelling concept known as "suspension of disbelief." The audience is at the edge of their seat with the rising tension of the story approaching its peak and just prior to climax, will accept just about any resolution the storyteller presents to be relieved of that tension. (See the ending of Jaws for a reality based example.) You'll find a lot of movie magic only works because the audience is able to suspend their disbelief.
It happened because it had to. Luke's the hero. You want an in-universe explanation: practice shooting womp rats and using the Force to aim. Case closed.
*Waits for innuendos associated with stages of storytelling*
The architects of this mod have the ability to change things as they see fit, provided the audience is willing to suspend their disbelief of any concept, both sound and outlandish.