After much discussion, I've made a change to the way the site calculates the TA scores for DLC (downloadable content).
Instead of being based on the number of gamers that have the game, the TA Ratio is calculated based on the number of people that have earned at least a single achievement in that DLC pack.
This has obviously reduced the TrueAchievement of DLC achievements (as not everyone who has the game also has the DLC, effectively making them less "relatively rare").
However, this seems like a much fairer way of scoring the DLC as it was definately artificially inflated previously.
The one problem we have is with single achievement dlc packs - these result in a TA RAtio of 1.00 as everyone who can be considered to have the pack also has the achievement (by definition). So in this rare case, we use the TA Ratio of the game (not including DLC packs) as the TA Ratio for the achievement.
It's not perfect, but it's much better than the previous system.
The other change that has happened recently is that your votes for a solution for an achievement that you yourself have also probided a solution for are no longer counted. This is to prevent people from simply adding a very similar solution to an existing one and at the same time voting the other one down in order to immediately promote their own solution to the top of the list.