but what is the logic behind this.
When you are trusting DefaultTrust, DefaultTrust is Depth 0, and Lauda is Depth 1 and someone whom Lauda trusts is Depth 2. You are trusting till Lauda's trust list.
When you put Lauda on your trust list, Lauda is Depth 0, Lauda's trust list is Depth 1 and their trust list is Depth 2. Therefore, you are trusting some more members.
The only logic is- when you are adding someone in your trust list, s/he becomes Depth 0.
Edit- You have forgotten to add a link to minerjones profile in OP.