Your prowess tracks your XP at your current age against an average of a very long history of all the previous players.  That history table is getting pretty large, so we will start pruning it to only compare you against a smaller set of people.  This means that your prowess may change because it is comparing it against different data, but this will not alter you, your character, its xp, or your progress in any way. 

We expect this change to happen this week, so if you notice a change in your prowess but havnt changed your playing style, you will now know why :)

It will compare against living and recently deceased (with the exception of really aged characters where there will not be that many to compare against, so it will keep records further back so it can maintain at least a minimum number of people to compare against)