This can happen if the Web address is slightly different, which often occurs when some tracking information has been added to the link. This typically occurs if someone has shared from another source such as twitter instead of directly from the main address where the article is hosted. To be honest it's only something I've been paying more attention to myself recently. When posting try and make sure the address to the article is clean and unnecessary parts are removed. I'm not sure if there is a technical fix to this and it might just be the case that "unclean" links will be less likely to get upvoted in the future.