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I'm not the expert on this but if there are missing blocks for the file, like if only part was downloaded and never finished, it needs to download them. If all the blocks are local then when you select download it will complete instantly and then you can go to the download thing and select retrieve.

You can have local URLs for files and not have the blocks stored in your cache, or never download any. URLs just identify a file, you still have to download the blocks.