These are the thumbnails of the same photo in my Camera Roll and Photo Stream:
Notice that the one on the left is “fuzzier”. Zoom in on the spots on the tablecloth or the text on the book cover for example.
All photos I take are the same. This is on an iPhone 8, with iOS 12.4.1 and I don’t use iCloud Photo Library.
Is there a technical reason for this, or is it simply a bug?