Welcome to Android Central! It can be confusing because if you move or copy a photo file, the creation date of that particular file is when you moved or copied it. I think most stock gallery apps will sort files by date created, not the date the photo was actually taken. If you moved or copied a whole lot of photos at the same time, then all the files have more or less the same creation date/time, and so the sorting order will tend to be all mixed up.
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