It isn't that it is specifically a version behind. For a while there it was on par (made it up to r5 on one version). The reason it takes a while to update to the latest is because a few of the utilities that are a part of the pack can take a while to update and the version before last updated quite a bit of the behind the scene stuff (emotions and what not) so both Therapist and DFHack took a good bit of time to update. At the start of this update cycle the pack would update with or with out those but since we are far enough along the curator of this pack has decided to wait for them. Eventually the pack will catch up with the current version though as the running joke states, when the pack updates it probably means a new version of vanilla will be coming in the next few of hours.