Why Does Explorer Show Two Different Dates for One Photo?
Because they are different things: when the file arrived, and when the photo was taken.
Explorer can show Date Created, Date Modified and Date Taken. The first two belong to the file on your disk. The third belongs to the photograph and is the only one that means anything about the picture.
After a download or a copy, the file dates are today while Date Taken is years ago. Both are accurate answers to different questions.
Sort by Date Taken, and fill it in when it is blank. Takeout JSON Metadata Fixer does the filling in. It runs on your own computer and is free for your first 500 photos.