I do not understand your point. If somethging always count as defended it is "just" defended, but always.
And all the units have to deal with that status. Always. Some of them deal by hitting on 5+ (and no charge bonus) some of them hitting on 4+ like cannon.
All defended targets, temporarily defended, occasionally defended and always defended targets are hit on 4+ by a cannon ball.
Defended is the status. It is not the probability to hit. The probability to hit is considered in completely different phase of the game. Let's say at the end of movement phase some units are defended, some not. Steam Tank commander does not have to worry about the cover - he is always defended. Then the Movement phase is finished. And then different units try to hit defended targets in the shooting phase.
Some of them are doing it on 5+ some on 4+.
And it is no generic versus special rule discussion here. That's how I see it.