I believe this can happen for a few different reasons. Typically it's because a delivery is somehow incomplete - usually because a photo started uploading but was deleted from the Eye-Fi card before it finished. As I understand it, our system assumes any file that started transferring from the card is one that you want us to finish delivering at some point, so it's accurately showing you the status of that upload.
On the occasions when I deleted some photos and experienced this issue, I just ignored it and kept shooting. I know why it happened, and once I've moved on to another day or month, I rarely see it again.
Sometimes you can remove these by right-clicking the image thumbnail (or Control-clicking on a Mac) and choosing to remove the item from Eye-Fi. Alternately, open a year or month in Eye-Fi Center, then right-click the card name and choose to remove all items for the Eye-Fi card.
If you've already done this, please right-click the image and choose More Info. What is the delivery status? Please send your Eye-Fi Helper logs to us at firstname.lastname@example.org
and we'll take a look. To find the logs: viewtopic.php?f=30&t=5776
and send us any files in the Eye-Fi application data folder called "EyeFi", "EyeFi.log" or "EyeFi.log.1".