The bullet loses enough speed from air resistance before it sinks below the aircraft’s flight path and the air plane flies into it essentially, as I understand it.

The rounds Attridge fired while traveling at 768 mph left their cannons at approximately 2,000 miles per hour. However, immediately after being fired, they encountered enough air resistance to produce significant drag. This drag resulted in a greatly reduced forward velocity, causing their trajectory to curve downward—directly into the flight path of the aircraft from which they had been fired. As the bullets descended and their speeds decreased to about 400 mph, the Tiger also descended but with an increased speed of 880 mph. Just as he began to pull out of his descent, Attridge was struck three times. The first bullet pierced his nose cone, the second went through his windshield, and the final one directly struck his right engine intake. The time between him firing the first rounds and taking the hits was a mere 11 seconds.

https://planeandpilotmag.com/grumman-f11-tiger-shoot-itself-down/

I imagine the bullet started out faster than the plane but, since it isn't actively propelled, it slowed down and the plane didn't. So less so that the plane was faster than the bullet and more that the plane was fast enough to catch up with the bullet before it fell to the ground. Edit: or just a totally made up tidbit, this is the internet after all

Plane shoots, plane dives then climbs in a reverse palabora. Bullet flies mostly straight

The plane shot the bullets upwards, then continued on a level path.

Once shot, the bullets take on a ballistic trajectory, i.e.: a parabole. On the downwards part of their trajectory, the bullets' path intersected with the plane's level flight path.