Benjamin,
The Lord's blessing!
I fear you are falling into the trap of the west and seeking to arrange by logic and rational inquiry those things which are of God. What makes an Ecumenical Council ecumenical is its inspiration under the Holy Spirit and acceptance across the Church as such. Now they bear certain marks which you have categorized, but if they lack the ultimate recognition across the Church as such, they aren't ecumenical. The Spirit does not move by the rules and syllogisms of man. So, if a council were to tread over previous ground, but be recognized by the Church as ecumenical, then it is. But asking why one is not ecumenical is a different question, and the retreading of previous ground would certainly be one reason.
Incidentally, I would not consider the 2nd Ecumenical council to at all be treading over new ground. The Spirit-fighters had not been previously addressed and let us not forget that they completed our Symbol of Faith in its present form. You might have a better argument on the 5th Council not covering new ground, but knowing the controversy of the Three Chapters was weighty enough that a Pope was anathematized and imprisoned in Constantinople might shed some light on why it was ultimately received universally.
In Christ,
the unworthy Irineos
