It means when having just "DefaultTrust" you appear with slightly more less trust than when having "DefaultTrust ~suchmoon". I haven't checked it in more detail.
One option is you excluded someone who left you positive trust. However after a quick look this doesn't seem to be the case.
The other option is you included someone who excluded a user who left you positive trust.

Because there's an exclusion in here                  ^^^, it's possible there's an error similar to the one noted by LoyceV. Exclusions and recursion can get things very complicated. I'll need to check to see if I can improve those numbers.

I don't think this "-1" is very meaningful. I just think this list can be used as base to identify DT1 users who have modified their own trust the most in order to further check those accounts.