How to use RF symmetrical spiral inductor ?

Attached file is a paper which has some discussions about the operation principle of differential inductor. All of us know that the Q factor under differential driven is almost twice than single-ended operation.Got few questions here.

1. Better Q results from the comparison of "differential inductor and single-ended inductor" or "differential inductor under single or differential driven?"

2.Any measurement difference between "differential signal driven" and "single ended driven" if center tapped open, I mean, they have the same current flows (same direction) in the inductor, isn't it? suppose we should see the same physics. Why people would say the Q is different?

3.Anyone could explain the physics of the difference under differential and single-ended driven? Don't you think the two port theory from "John Long" for explaining Q factor doesn't make too much sense, especially for someone wants to understand the real difference of differential inductor ?