When you exclude someone to see the difference, you see the differences on DT2.
However, when you include someone, you put him at Depth 0, and you see not only his Depth 1 (which would be DT2 if the user is on DT1), but also his Depth 2 (which would be DT3 if the user is on DT1).
So for the excluded DT1 users, you're effectively comparing their "would be" Trust score to DT3 instead of DT2.
You're absolutely right. My bad.
What would be the right way to do it? Add the user (i.e. OgNasty) into the trust list on one account and then use a second account to add the first one into the trust list so that OgNasty goes down one level, right?
If I do that I get these numbers for him: 23: -6 / +73
Edit: I've updated OP with this recalculation.