There are certainly groups who sacrifice quality to minimize file size. Sounds like you downloaded a file from one of those. There are people out there who care so much about quality they will splice different sources together because some specific section is better in some obscure release for some reason. You also have blu-ray remuxes if you want the exact original.

Netflix manages to generate compression artifacts in 90% black frames, to say nothing of highly dynamic footage. Satellite TV is even worse, it actually hurts to watch. Makes me feel like an idiot for actually paying for this.