The version number used by Document.getVersion() and inserted in the metadata of the generated pdf files is modified to append a build number to identify each snapshot. For instance, as of this writing, the latest version is "iText 2.1.0-SNAPSHOT build 3182 (by lowagie.com)".
Using the latest link will always redirect you to the latest version available, which is currently "5.0.4_SNAPSHOT build 4562".
Note that the snapshots should be used mainly to validate recent bug fixes and should not be used on production systems as they are after all snapshots and there is no guarantee as of the stability of the builds...