What counts as current-carrying
This is the part people get wrong. Equipment grounding conductors do not count. Neutral conductors that carry only the unbalanced current of a normal balanced multiwire circuit generally do not count either - but a neutral on a circuit feeding substantial nonlinear load does count, because harmonic currents put real load on it.
The count is also per raceway, and only applies where the bundle runs more than 24 inches. Short nipples between enclosures do not trigger it.
Why bundling matters
Each conductor in a bundle is a heat source, and the ones in the middle have nowhere to shed it. Nine conductors in a pipe heat each other to the point that each carries only half its table value. This is also why conduit fill is limited: fill and derating are two expressions of the same thermal problem.
Combine this with the temperature correction - a hot attic full of conductors gets both, multiplied, and the result can be startling.