That might have been the version that originally came with that computer and is the most recent for version 5.
I don't know why iPhoto thinks the library was opened with a newer version. You or someone else could have used a newer version before, or it might be a program error. In the current version of iPhoto there is a file called "AlbumData.xml" in the Library folder which right at the begging says which version of iPhoto it was created with under "Application Version". If you look at this file (if it exists for the older libraries as well, I don't know) you can tell which is the minimum iPhoto version you need to open this library. Don't edit this file; you might break your library.