Is it possible that those photos had geotags in the file? If they were exported from iPhoto with an earlier location, or came from an iPhone or other device that geotagged them, iPhoto will not let you get rid of the location from its interface.
If it happens again, you tell by right-clicking them in iPhoto and choosing "Show File" (or "Show Original File" if it is available). Then open them in Preview and use Tools > Show Inspector. If the More Info tab has a GPS tab with a map, then there is a geotag in the file itself.