IIRC, it is not actually wrong, but really whether the angle is correct or not comes down to whether you want to copy the proportions of the studio filming model or full size set blue prints.

The MPC/AMT kit is actually based on the full size set blueprints, and I believe it accurately represents those as far as the angle and proportions as the set had a steeper angle and top fin shape.

The filming model on the other hand was different in that respect.

Maybe it was due to foreshortening for the camera since it was only seen from the front or some other limitation, or maybe since the filming model and full size set were done by separate groups in California and England and the ILM modelers took a little liberty based on their own limitation, although I do not know for sure

Revell actually used the same blue prints for the source of their kit, and it has the same angles and proportions of the MPC/AMT kit

The X-Wing game miniature and Kenner Shuttles on the other hand, follow the filming models profile

i.e. in the end, technically either way is correct, just depends on which source it represents