I hear that SaltySpitoon fella is pretty solid as well  but joking aside, the quality of a person's feedback and the users it is from are more important than some numerical score. Having a +10 from Theymos is probably more telling than +100 from 10 random people. At a certain point, I really don't think the number matters all that much.

It is an interesting metric, but there are a lot of reasons why you can't necessarily draw conclusions from this alone. For example, each person's marketplace presence. Phillipma has a pretty large presence in the computer hardware section (if I'm not mistaken, I'm not looking up post history now, just recalling this from memory), Ognasty and Blazed in the Collectibles sections, myself in the Goods/Collectibles sections, and the others not as much. The 15-20% that you are calling normal seems to be about the standard for those not as heavily involved in the marketplace sections, so what does that mean about people who are getting tons of feedback for trades they are doing?

As you said in your OP, there is also some consideration to the nature of how a relationship starts. I have a couple of people on my trust list that I would never have known had I not traded with them, so to a certain extent its inevitable. The real metric for who you should trust to add to your trust list, are people that you believe will leave accurate feedback for others and act fairly.

And finally, usergroups kind of just establish themselves in boards where people frequent. The Alt coin sections have pump/dump groups, the ICO hunter groups, etc. Another example is the weird cult of people that have established themselves in the collectibles section. If you find yourself in one of those groups, by interacting with them, you'll probably find people you trust to add to your trust list. More so than just following the posts of a user you agree with in the speculation section. If that group was formed in a section that facilitates trades, your % I'd imagine would get higher.

Phillipma is indeed a large outlier, but this thread serves as the smoke to warrant further investigation, not necessarily something to draw conclusions from. I'm really tired so ignore the spelling and grammar mistakes, I write walls of text and proofreading is more than I'm up to right now.