Its evidence, but not actually meaning there is political power. Just like there is evidence in a case, but it doesn't mean the person is guilty. Political power is the power to influence politics, which means that high-level positions it is easier to do generally, but does not mean that ruler always actually has the most political power or that a knight can't be powerful politically either.
How many vassals they have doesn't give them political power. One of my characters is very often on opposite sides on issues so the fact that I am his vassal does not add to his political power. Another character of mine is only a knight albeit an ambassador as well, and has a pretty decent amount of political power. More vassals /= more political power. For some it will, because they have very good relationships with their vassal, but it doesn't apply to everyone.
I have to strongly disagree with you. The power to command armies, enforce laws, control trade, expend treasure and set tax rates are the prerogatives of those holding political offices. Individuals can be more influential than their official titles suggest. There's what's sometimes called the "power behind the throne," e.g. a trusted adviser who exercises power by virtue of his influence over the holders of political offices, formally or informally. If you can think of an effective formula to measure that, I'll add it to the feature request.
