1. Not pixel-perfect duplicate.
1=>2. Some pixels different.
3. Having better color sampling (y cb cr sub 4:4:4 against 4:2:0) and less exif loss.
1+2=>3. This one has better quality than previous uploaded, even slightly.

Fedayeen_Beyaatres said in comment #2630751:

1. Not pixel-perfect duplicate.
1=>2. Some pixels different.
3. Having better color sampling (y cb cr sub 4:4:4 against 4:2:0) and less exif loss.
1+2=>3. This one has better quality than previous uploaded, even slightly.

Bluesky often recompresses their image, so the original yuv420 input got turned into a "higher" JPEG quality yuv444 image, but the level of detail is lower (since it's double compressed). You can zoom in and see that the same JPEG artifacts are present in both images, which you wouldn't expect if this one is actually less compressed.