Interesting observation DTB. I hadn't noticed that
Have had a quick look at the numbers and will put forward two reasons:
1/ The effect of age, particularly as players pass 55. There are a lot of people who have been on the Senior PGA Tour a long time! They are not playing as well as they have done a few years ago.
2/ There have not been a lot of 'star' players entering the Tour this year. As a result, my 'field' weight per event has increased by an average of just over 73 in the last few years on this Tour to just over 74. Basically, a reduced entry of 'name' players with a low Sagarin score and the same players playing week after week and getting older.
Of course, it could be human error in the number-crunching department