Oswald qualified as "Sharpshooter" and later his qualification fell to "Marksman".

In the reconstruction, they used a shooter with similar ability to Oswald.

If you've tried shooting at moving targets, you'll know that the degree of difficulty is greatest when the target is moving across your field of vision (90º) and the difficulty reduces as the angle reduces.

Oswald was shooting at such a reduced angle that the degree of difficulty was in his favour to make the shot(s).