Were you on a phone? Image hosts generally recycle urls but set headers so it doesn't mess you up. Most phone companies put a sort of CDN between you and potentially large assets (pictures, videos) and they're not always sensitive to that behaviour. They'll try to get away with never making a second request to the content provider if they can get away with it and continually serve you from their stale cache.
So that's a possibility. Maybe also your browser has a really janky hash table implementation where a hash collision isn't handled by open addressing or something and images that hash the same will all get the same cache hit. That's even less likely though so IDK.
