Thanks for letting me check out the model. I see Anvart and ref basically already answered it. In SH4 some models have vertex normals (aircraft, f.ex), but most don't.
I like to add to what already is said, that if a model doesn't have normals, what the game engine does (and so does S3D, it's using the exact same technique), is call a 3D-function "ComputeNormals". This function smoothens out the model as ref already indicated.
You will see that if you choose to import that same model (unmodified), and tick 'Import Normals' in S3D, you will get a close to exact same rendering as your first screenshot. Using precomputed normals has the benefit of proper lighting calculations and faster loading (no calculation needed), but increases the overall filesize (one normal-vector per vertex extra) and possible memory usage (depending on how it's implemented in the game). Not only that, I am currently unaware if the game accepts precomputed normals for each and every model (as I never tested this).
In any way, that's probably why you got different renderings, you looked at the same model with and without normals at different moments...
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