That is the version of the X.Org xserver binary. However X.Org consists of many more programs than just the X server. See http://www.x.org/wiki/Releases/7.5
for an overview.
Actually, Slackware 13.1's version of X.Org is somewhere between 7.5 and 7.6. The X.Org 7.5 release was in september last year, which would make it pretty stale for inclusion in Slackware 13.1, so a lot of the individual X.Org programs were updated to their latest release.