Why does the streaming quality look different from the download?

Because they are different files, by design. The stream in the gallery is an optimised playback rendition built to start instantly and play smoothly on whatever connection the client has. The download is your original master, untouched.

Any online player, including DLVRD's, adapts to the viewer's device and bandwidth. On a fast connection and a good screen the stream looks excellent. On a phone on a train it steps down to keep playing rather than buffering. That adaptive behaviour is what makes online delivery practical for large films.

None of it affects the deliverable. When the client downloads, they receive the exact file you uploaded, at its full bitrate and resolution.

If a client questions quality after watching the stream, point them at the download. That is the film; the stream is the preview.